Green groups are demanding the government immediately deal with diesel pollution pumped out by thousands of buses and trucks with substandard engines.

The Hong Kong Sustainable Development Forum, a non-profit concern group, said Thursday the government should move quickly.

``We are very disappointed by the audit report's findings, [which confirm] diesel fuel exhaust is a major contributor to inner city pollution [and that, together with particulate matter, form] a very serious health hazard,'' forum member John Herbert told The Standard.

The groups spoke up one day after the release of an Audit Commission report criticizing Transport Department measures to control the emission of hazardous chemicals from the 130,000 diesel-fueled vehicles in the territory, confirming them as the main contributors to air pollution.

``Far too many diesel vehicles'' are allowed to ``splutter ... particles'' into the atmosphere, Herbert said.

Both Greenpeace and Clear the Air said the government has not done enough to help install cleaner engines on diesel vehicles, especially buses.

Clear the Air chairwoman Annelise Connell said public buses, which have for decades been a major transport mode in Hong Kong, are the main generators of hazardous chemicals.

She said most buses have older-model diesel engines.

``There is no way to get these buses off the road if you replace the buses so slowly,'' Connell said.

Since 2000, the government has allocated at least HK$721 million to subsidize installation of emission reduction devices in older diesel vehicles, most of which were registered before 1995.

Chow Sze-chung of Greenpeace said the government could do more to encourage the upgrade of bus engines, besides the installation of emission reduction devices.

``If the government can do a better job on buses, the situation will be much improved,'' he said.

KMB, the largest public bus operator in the territory, said its fleet's emissions have been reduced by about 75 percent since 1992 when the operator began shifting to cleaner engines. But Connell said nearly 50 percent of the KMB fleet has not been fitted with cleaner engines.

The figure could not be verified with KMB Thursday night.

Despite all this, the Transport Department said Thursday the number of diesel vehicles failing annual smoke tests has decreased dramatically.

This came after the Audit Commission said lenient standards and cheating were the major causes for a low detection rate of smokey vehicles.

Most taxis and minibuses have shifted to clean fuels, but about a third of vehicles, including most of the territory's 6,000 buses, lorries and goods vehicles, still use the smokey fuel that contribute about 90 percent of hazardous chemicals generated from road traffic.

The department said the number of diesel vehicles that failed annual vehicle inspections decreased from 610 in 2003 to 385 last year.

It said the decline is an improvement that is attributable to the installation of more particulate removal devices on vehicles.