BANGKOK (REUTERS) - Thai authorities are trucking drinking water to parts of Bangkok and urging residents to shower less as a worsening drought and rising sea levels increased salinity, a growing risk faced by many Asian cities, climate researchers said.

Bangkok's water authority said the capital's tap water was becoming saline as seawater pushed up the depleted Chao Phraya river, a source of much of central Thailand's water.

Prime Minister Prime Minister Prayut Chan-o-cha this week asked the public to save water by taking shorter showers.

Making matters worse, Thailand's dry season began in November and usually lasts through April, but this year, the authorities said it could last until June and drought has been declared in 14 provinces.

The drought conditions have worsened saltwater intrusion, which can have major impacts on farming and on health as drinking water is contaminated, said Mr Suppakorn Chinvanno, a climate expert at Chulalongkorn University in Bangkok.

"It is becoming a more serious issue, with the intrusion coming farther inland this year and earlier in the season," he told the Thomson Reuters Foundation.

"It will have a serious impact on agriculture in the region, as rice is a very water-intensive crop," he said.

Many of Asia's biggest cities, including Mumbai, Shanghai, Bangkok, Ho Chi Minh City and Jakarta, are coastal and low-lying, making them vulnerable to rising sea levels and extreme climate events such as more frequent and deadlier cyclones.

Indonesia plans to move its capital to the island of Borneo, as Jakarta - on the north coast of Java island - is slowly sinking and suffers regular flooding.

Cities located in deltas have to increasingly deal with saltwater intrusion, said Ms Diane Archer, a research fellow at the Stockholm Environment Institute in Bangkok.

"The reality of climate change and sea level rise means this is increasingly going to become an issue for delta cities as sea water intrudes into rivers and aquifers, especially during drought and where groundwater is already depleted," she said.

"Bangkok is controlling groundwater extraction but is suffering from subsidence, making it more vulnerable to sea- level rise. As sea levels continue to rise, it is likely that salinity is going to become a growing threat," she said.

Elsewhere in the Mekong Delta, saltwater intrusion is already a problem, with some cities in Vietnam monitoring salinity levels to alert farmers on whether the water is safe for irrigation, Ms Archer said.

"Farmers may need to adapt their crops to those more suited to brackish water," she said.