PICTURES

Tourist warning as smoke reaches Yosemite valley

El Dorado Hills firefighters takes a break from battling the Rim Fire on Aug 28, 2013 near Groveland, California. Over 3,500 firefighters are battling the destructive Rim Fire that has charred 180,000 acres and has entered a section of Yosemite Natio
El Dorado Hills firefighters takes a break from battling the Rim Fire on Aug 28, 2013 near Groveland, California. Over 3,500 firefighters are battling the destructive Rim Fire that has charred 180,000 acres and has entered a section of Yosemite National Park. -- FILE PHOTO: AFP
Smoke rises from a fire approaching through the forest near cattle at the Rim Fire just outside of Yosemite National Park, California, on Aug 28, 2013. One of the largest California wildfires on record roared deeper east into Yosemite National Park o
Smoke rises from a fire approaching through the forest near cattle at the Rim Fire just outside of Yosemite National Park, California, on Aug 28, 2013. One of the largest California wildfires on record roared deeper east into Yosemite National Park on Tuesday, frightening away many late-summer visitors, while the sprawling blaze also crept closer to thousands of homes west of the park. -- FILE PHOTO: REUTERS
Scorched roadside reflectors stand near a smoldering forest at the Rim Fire just outside of Yosemite National Park, California, on Aug 28, 2013. One of the largest California wildfires on record roared deeper east into Yosemite National Park on Tuesd
Scorched roadside reflectors stand near a smoldering forest at the Rim Fire just outside of Yosemite National Park, California, on Aug 28, 2013. One of the largest California wildfires on record roared deeper east into Yosemite National Park on Tuesday, frightening away many late-summer visitors, while the sprawling blaze also crept closer to thousands of homes west of the park. -- FILE PHOTO: REUTERS
Cows graze in a field as the Rim Fire burns in nearby hills on Aug 28, 2013 near Groveland, California. Over 3,500 firefighters are battling the destructive Rim Fire that has charred 180,000 acres and has entered a section of Yosemite National Park.
Cows graze in a field as the Rim Fire burns in nearby hills on Aug 28, 2013 near Groveland, California. Over 3,500 firefighters are battling the destructive Rim Fire that has charred 180,000 acres and has entered a section of Yosemite National Park. -- FILE PHOTO: AFP
Dust from a tree felled by firefighters rises over Highway 120 at the Rim Fire just outside of Yosemite National Park, California, on Aug 28, 2013. One of the largest California wildfires on record roared deeper east into Yosemite National Park on Tu
Dust from a tree felled by firefighters rises over Highway 120 at the Rim Fire just outside of Yosemite National Park, California, on Aug 28, 2013. One of the largest California wildfires on record roared deeper east into Yosemite National Park on Tuesday, frightening away many late-summer visitors, while the sprawling blaze also crept closer to thousands of homes west of the park. -- FILE PHOTO: REUTERS
The Rim Fire expands in the hills behind Tuolumne City north-west of Yosemite National Park, California, on Aug 28, 2013. Yosemite National Park, faced with the spread of a massive California wildfire, extended a key road closure in the park on Wedne
The Rim Fire expands in the hills behind Tuolumne City north-west of Yosemite National Park, California, on Aug 28, 2013. Yosemite National Park, faced with the spread of a massive California wildfire, extended a key road closure in the park on Wednesday in a move expected to hinder visits to one the nation's top outdoor destinations over the Labor Day weekend. -- FILE PHOTO: REUTERS
The Rim Fire burns at night in this undated United States Forest Service handout photo near Yosemite National Park, California, released to Reuters on Aug 30, 2013. Fire crews battling to outflank a monster wildfire inside Yosemite National Park made
The Rim Fire burns at night in this undated United States Forest Service handout photo near Yosemite National Park, California, released to Reuters on Aug 30, 2013. Fire crews battling to outflank a monster wildfire inside Yosemite National Park made headway on Friday in confining flames to wilderness areas but were powerless to salvage the region's sputtering tourist economy at the end of its peak summer tourist season. -- FILE PHOTO: REUTERS
A tanker drops retardant on the Rim Fire in this undated United States Forest Service handout photo near Yosemite National Park, California, released to Reuters on Aug 30, 2013. Fire crews battling to outflank a monster wildfire inside Yosemite Natio
A tanker drops retardant on the Rim Fire in this undated United States Forest Service handout photo near Yosemite National Park, California, released to Reuters on Aug 30, 2013. Fire crews battling to outflank a monster wildfire inside Yosemite National Park made headway on Friday in confining flames to wilderness areas but were powerless to salvage the region's sputtering tourist economy at the end of its peak summer tourist season. -- FILE PHOTO: REUTERS
This Friday, Aug 30, 2013 image provided by the US Forest Service shows a member of the BLM Silver State Hotshot crew using a drip torch to set back fires on the southern flank of the Rim Fire in California.  -- FILE PHOTO: AP
This Friday, Aug 30, 2013 image provided by the US Forest Service shows a member of the BLM Silver State Hotshot crew using a drip torch to set back fires on the southern flank of the Rim Fire in California.  -- FILE PHOTO: AP
In this Friday, Aug 30, 2013 image provided by the US Forest Service, members of the BLM Silver State Hotshot crew perform burn operations on the southern flank of the Rim Fire in California.  -- FILE PHOTO: AP
In this Friday, Aug 30, 2013 image provided by the US Forest Service, members of the BLM Silver State Hotshot crew perform burn operations on the southern flank of the Rim Fire in California.  -- FILE PHOTO: AP
In this Friday, Aug 30, 2013 image provided by the US Forest Service, a member of the BLM Silver State Hotshot crew patrols the fire line during burn operations on the southern flank of the Rim Fire in California. -- FILE PHOTO: AP
In this Friday, Aug 30, 2013 image provided by the US Forest Service, a member of the BLM Silver State Hotshot crew patrols the fire line during burn operations on the southern flank of the Rim Fire in California. -- FILE PHOTO: AP
The late afternoon sun shines through smoke as fire creeps through the forest at the Rim Fire just outside of Yosemite National Park, California, on Aug 27, 2013. Smoke from the huge wildfire in Yosemite National Park reached the heavily-touristed he
The late afternoon sun shines through smoke as fire creeps through the forest at the Rim Fire just outside of Yosemite National Park, California, on Aug 27, 2013. Smoke from the huge wildfire in Yosemite National Park reached the heavily-touristed heart of the park Saturday, officials said, warning visitors against strenuous activity. -- FILE PHOTO: REUTERS

SAN FRANCISCO (AFP) - Smoke from the huge wildfire in Yosemite National Park reached the heavily-touristed heart of the park Saturday, officials said, warning visitors against strenuous activity.

Webcams showed smoke apparently clouding the world-famous Yosemite Valley, the spectacular area in the middle of the California park visited by millions every year.

More than usual visitors were expected in the park for the Labor Day weekend, although officials had said the increase could be smaller than usual due to the so-called Rim Fire, which started two weeks ago outside the park.

"Heavy smoke is now visible south of the Tioga Road, including in Yosemite Valley," said the park website's latest update after a change in wind direction from the blaze.

"Visitors to the area should avoid extended strenuous physical activities outdoors. Additionally, those (..) sensitive to air quality impacts should avoid going outside in Yosemite," it added.

Despite the warning, officials said they were optimistic of making further gains on the blaze, known as the Rim Fire, but warned that hot, dry conditions continued to create a challenging environment.

"We're hopeful that we are going to turn the corner, but it's hot, it's dry, and there is a westerly wind," US Forest Service spokeswoman Leslie Auriemmo told AFP. "There's a lot of fuel out there. We remain in a high state of alert." According to latest figures early Saturday, the fire has burned 888 sq km and continues to threaten 4,500 structures.

A total of 4,995 firefighters have been deployed to battle the flames, which have so far destroyed 11 homes and 97 outbuildings.

The fire, which started on Aug 17, was 35 per cent contained as of Saturday, up from 32 per cent on Friday.

Yosemite National Park officials insisted on Friday that the fire posed no threat to tourists heading to the landmark destination on a busy US holiday weekend.

The flames remain some 24km from Yosemite Valley, the tourist heart of the park where millions of visitors flock every year to see majestic scenery such as the Half Dome and El Capitan rock formations.

"The area where it's burning right now is mostly wilderness... There's nothing in that location that would potentially be a safety issue," said Yosemite spokeswoman Kari Cobb.

Meanwhile investigators are looking into whether an illegal marijuana farm may have triggered the blaze, US media reports said.

Several reports quoted Todd McNeal, a local fire chief in Twain Harte, one of the towns affected by the 88,630ha inferno, said investigators had not pinpointed the cause of the blaze.

"We don't know the exact cause," McNeal was quoted as telling a community meeting. However, he added it was "highly suspect that there might have been some sort of illicit grove, a marijuana-grow-type thing." "We know it's human caused. There was no lightning in the area," he said.

US Forest Service officials say the cause of the fire remains under investigation.

The San Jose Mercury News reported that authorities in California have faced increasing problems with marijuana farms hidden deep in the region's rugged wilderness.