An advisory council under the Environment Agency proposed Monday a set of stricter emissions guidelines for diesel automobiles that would lower the upper exhaust limits of nitrogen oxide and particulate matter by more than one-fourth by 2004.

Automakers and oil refiners are also requested to halve the 2004 emission level by 2007.

Meanwhile the same day, a Transport Ministry council formally approved a plan to require automakers to significantly improve efficiency of both diesel and gasoline vehicles by 2005 and 2010, respectively.

Automakers will have a difficult time meeting targets for both exhaust gas and fuel efficiency at the same time, experts say, citing technical problems.

According to the new Environment Agency guidelines, the upper limits for nitrogen oxide and particulate matter emitted from diesel cars should be lowered by between 25 percent and 35 percent between 2002 and 2004. The target and reduction times differ according to weight and automobile type.

Nitrogen oxide and particulate matter are believed to cause respiratory organ diseases. Of 382 air pollution measuring sites along roads across the country, only 251 meet environmental standards set by the national government in 1997, according to the agency.

The agency has already moved to lower by about 70 percent the amount of nitrogen oxide, hydrocarbon and carbon monoxide new gasoline automobiles can emit by 2000.