In a letter to Bush, 20 senators also called for American leadership to tackle climate change, and voiced support for G8 host Germany's plan for concrete steps to tackle the threat.

The senators upped pressure on Bush days after he announced plans for a "new framework" in which the world's biggest carbon polluters would set long-term goals for curbing greenhouse gases.

The plan was criticized by some Bush political opponents, and in some foreign capitals, as not going far enough.

"President Bush has offered promising rhetoric, but has been short of real commitments and specifics," said Senator Joseph Biden, chairman of the Democratic-led Senate Foreign Relations Committee.

"I hope the US can be a consensus builder in this week's G8 discussions, not a roadblock," Biden said in a statement accompanying the letter.

Republican Senator Olympia Snowe voiced support for climate change proposals framed by German Chancellor Angela Merkel.

"I believe that Chancellor Merkel has proposed a constructive framework that produces tangible results and it is incumbent on the President to either work within this framework or propose an alternative," said Snowe.

"Postponing this issue for a future summit would be a disservice in establishing American leadership on this critical issue and I implore the President to rise to this momentous occasion.

Germany has called for a statement limiting worldwide temperature rise this century to two degrees Celsius (3.6 degrees Fahrenheit) and cuts to global greenhouse emissions to 50 percent below 1990 levels by 2050.

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