BEIJING - MICHAEL Phelps scorched to his sixth gold and his sixth world record of the Beijing Olympics on Friday in a great morning for the United States in the pool and the gymnastics arena.

North Korean Kim Jong Su, who won silver and bronze in shooting, became the first medallist to test positive for drugs, and organisers said his medals had been rescinded.

Arch-rivals the United States had a much better day, three golds in the Water Cube and one in women's gymnastics to break China's dominance there.

Leading the way was Phelps, who now has 12 career Olympic golds, three more than anyone else.

He is closing in on Mark Spitz's 1972 record of seven golds at a single Games - all of which also came in world best times.

The 23-year-old showed little reaction apart from a quick shake of his fist after victory in the 200 metres individual medley on Friday but burst into a broad smile on the podium later.

'I just wanted to step on it in the first 50 a little bit and try and get out to an early lead,' said Phelps. 'I knew in the first half if I got a big enough lead I thought I could hang on and that's all I wanted to do.'

He races in the 100 butterfly final just after 10 am on Saturday morning, and if all goes well could be in the running to break Spitz's record in the 4x100 medley relay final, to be held just before 11 am in the Water Cube on Sunday.

In the next-door Bird's Nest stadium, athletics action finally began on Friday under blue skies, with Usain Bolt, Asafa Powell and Tyson Gay coasting to victory in their heats of the men's 100 metres.

China had dominated in gymnastics in the first week, winning the first three golds on offer. But the US team made up for disappointment in the women's team event by winning gold and silver in the individual all-round event on Friday.

Nastia Liukin fought back the tears after winning gold, ahead of compatriot Shawn Johnson and China's Yang Yilin.

In the pool it was also America's morning. Ryan Lochte set another world best time as he won the men's 200 backstroke, ahead of compatriot Aaron Peirsol.

'That's the theme of the meet, you have to break a world record to win,' Peirsol said.

Team mate Rebecca Soni also won the women's 200 metres breaststroke in a world record time, coming from behind to overtake the 100 metres winner Leisel Jones from Australia.

Two years ago she had minor heart surgery.

'It's been a long road to get here. I cannot believe what has just happened. I've won a gold medal,' said the 21-year-old. 'I was a little bit in shock. I was shaking a little bit up there.'

It was a double disappointment for Australia's women, dominant in the pool until now, when world champion Libby Trickett was also overhauled right at the finish by Germany's Britta Steffan in the 100 freestyle.

The quest for dominance in the medals table is proving a fascinating contest between the United States and China, which came second in Athens in 2004. By midday on day seven, China led 22 golds to the Americans' 14.

The Communist Party is desperate to underline the country's growing superpower status by overtaking the United States, and its athletes has been playing to strengths in events like diving, table tennis and weightlifting.

Ethiopia carried African hopes
The gap between the top two may narrow when track and field gold medals are decided in the Bird's Nest stadium, and Friday's shot put contest gives the US squad a great chance for a morale-boosting counterattack.

It has been 48 years since one nation swept medals in that event, but US trio Adam Nelson, Reese Hoffa and Christian Cantwell have the skill to match their 1960 compatriots.

Medals will also be awarded on Friday in the women's 10,000 metres, where Ethiopia's runners dominate and could bring some cheer to Africa which has had a disappointing Games so far.

Tirunesh Dibaba, who has two world championship titles, is favourite. She expects to run the 5,000m too and hopes to become the first woman to win the Olympic distance double.

Olympic organisers have been waging a campaign against drugs cheats and that effort claimed two more victims on Friday. North Korean shooter Kim tested positive for a beta-blocker, a drug misused to decrease heart rate and stop trembling.

Vietnamese artistic gymnast Thi Ngan Thuong Do was also excluded from the Games after testing positive for furosemide, a diuretic which can be used as a masking agent for other drugs.

Security has been gradually ratcheted up at the Games since the father-in-law of the American volleyball coach was stabbed and killed in Beijing on the first full day of the Games.

X-ray and scanning machines have now been deployed at the most heavily visited section of the Great Wall just outside Beijing, an official said on Friday.

Sporadic protests have continued in Beijing despite the clampdown. Foreign activists unfurled a banner proclaiming 'Free Tibet' over an Olympics poster at the newly built headquarters of China's state television broadcaster on Friday. -- REUTERS