The pact, adopted unanimously among the 36-member council of the UN body, comes amid growing concern that the expansion of budget airlines and global air travel poses serious environmental risks.

Airlines will have until 2008 to comply with the new restrictions, the Montreal-based ICAO said in a statement.

The new nitrogen oxides standards are "12 percent more stringent than the previous levels agreed to in 1999," it said.

The council's move followed recommendations made by the 35th session of the 181 member ICAO last year.

Reductions in emissions will be achieved mainly through modifications to aircraft engines.

But some airlines have argued, however, that the move will entail extra costs which could threaten the fragile recovery of the industry which slumped following the September 11 attacks in 2001, and the SARS epidemic in Asia and Canada.

Aviation fuel releases greenhouse gases like carbon dioxide when it is burnt which contribute to global warming.

Nitrogen oxides and substances also produced are blamed for depleting the ozone layer, which filters the Sun's harmful rays.

The Kyoto Protocol on climate change, which came into force last month, called on industrialized countries to work through the ICAO to reduce greenhouse gases on civil aviation.

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