By Richard Black
Environment correspondent, BBC News website
US climate legislation faces an uncertain future
The UN climate convention says nations signing up to the accord reached at last month's summit will not have to do so by the deadline of 31 January.
The "Copenhagen Accord" asks countries to send figures by the end of the month on how much they will curb emissions.
But amid uncertainty over who is going to sign up, climate convention head Yvo de Boer said the deadline was "soft".
He said the Copenhagen summit had not delivered the "agreement the world needs" to address climate change.
His comments will come as a disappointment to campaign groups, who would like to see a firm timetable for further talks and political moves pursued through the year.
There is also some concern in "green" circles about the election of Republican Scott Brown to succeed Democrat Edward Kennedy as Massachusetts Senator.
Some campaigners fear this will delay, weaken or derail the progress through the Senate of the Boxer-Kerry bill on limiting carbon emissions, and could induce wavering supporters of the legislation to jump ship.
Earlier in the week, North Dakota Senator Byron Dorgan, a Democrat opposed to the draft bill, predicted that the Senate "will not do a climate change bill this year, but we will do energy".
Mr Brown has spoken against measures to cap US greenhouse gas emissions. One Greenpeace campaigner described his election as "definitely bad news".
However, Alden Meyer from the Union of Concerned Scientists told BBC News it did not necessarily signal major problems ahead for the legislation.
"It was already clear that we would need some Republicans [to support the bill], because some Democrats have said they wouldn't support cap-and-trade anyway," he said.
It appears that despite any uncertainty over domestic legislation, the US will send in its commitments on reducing greenhouse gas emissions to the UN climate convention secretariat by the end of the month.
This coming weekend, the BASIC group of Brazil, China, India and South Africa are due to consider their response.
As developing countries, the four will not commit to emission cuts but are supposed, under the accord, to detail what measures they will take to curb emissions growth.
There were signs that despite playing a leading role in writing the accord, they might decide not to endorse it. But sources now predict that all four will send in their plans, though they might not be as ambitious as the intentions they revealed before Copenhagen.
The EU has also indicated it will submit figures and support the accord, even thought it falls short of the "minimum ambition" the bloc was looking for in Copenhagen.
However, many other countries are known to have grave doubts about offering any endorsement of what they regard as a fundamentally flawed document; and at the end of the Copenhagen summit, several, including Bolivia, Cuba and Tuvalu, indicated they would not support it.
Mr de Boer described the accord as a "political letter of intent".
He said that policymakers were now in a "cooling-off period" before beginning discussions on what they might want from this year's UN climate summit, to be held in Mexico at the end of the year.
"The window of opportunity we have to come to grips with this issue is closing faster than it was before," he said.