"The United States should, at a minimum, refrain from blocking or obstructing such discussions ... since it would be inconsistent with its ongoing treaty obligations," senators said in a letter to the White House.

Last week, Harlan Watson, the chief US climate negotiator, said he was opposed to such discussions.

The treaty referenced is the United Nations Framework Convention on Climate Change, which set the stage for Kyoto.

Kyoto obliges about 40 industrial nations to cut emissions of greenhouse gases by 5.2 percent below 1990 levels by 2008-12.

The Clinton administration signed onto the framework agreement, but President George W. Bush pulled the United States out of Kyoto in 2001, saying it wrongly excluded developing nations and would cost American jobs.

Mandatory carbon limits imposed by the European Union have spawned a growing market for trading emission-reduction contracts.

Senators said it is just a matter of time before the US Congress enacts similar mandatory cuts.

"The Senate intends, at some future date, to require a program of mandatory greenhouse gas limits and incentives for the United States," senators wrote. "It is only a matter of time before Congress takes such action."