Galvanised by his recent trip to New York for a summit of large cities concerned with pollution, Governor Apirak Kosayodhin launched a five-year action plan starting this year to improve the city's environment and lessen pollution.

Bangkok's 5.5 million cars would be regularly checked for the level of carbon dioxide they emitted, he said, with heavy polluters made to improve.

The Bangkok Metropolitan Administration (BMA) had started working with gas stations to recycle used cooking oil into biodiesel, he said, with one station already equipped with the environmentally friendly fuel.

"As cars are one source which causes pollution, the BMA will help facilitate using biodiesel at gas stations," Apirak told reporters.

Waste management systems would also be improved as part of the plan, while buildings would be made to switch to more energy-efficient technologies, such as energy-saving lightbulbs.

"This will help save 20 to 50 percent of energy," he said.

Apirak said the city would start with BMA offices, which would soon have an environmentally friendly overhaul.

Bangkok, with a population of 12 million, spews more than 26 million tonnes of carbon dioxide into the air each year, nearly 20 percent of Thailand's total.

Its congested streets used to be heavily polluted, but residents have reported an improvement in the past few years as air quality policies improved and taxis switched from petrol to gas.

Leaders from 46 of the world's largest cities -- from Cairo to Shanghai and Los Angeles to Bangkok -- met in New York last week and called on their counterparts to unite to tackle climate change.

At the conference, 16 of the cities including Bangkok, London and New York signed up to a global warming initiative which aims to slash energy consumption through improved technologies.

The Thai capital was earlier this month host to a meeting of the UN's top body on global warming, the Intergovernmental Panel on Climate Change.