By Richard Black
Environment correspondent, BBC News website, Barcelona
Climate protesters say politicians are failing to show the right leadership
At least 40 world leaders are likely to attend December's UN climate summit in a bid to secure a new global treaty.
Some observers say only intervention from heads of state and government can close the deal, given the gulf between industrialised and developing nations.
Others maintain the harsh words bandied here at the final preparatory meeting amount to no more than posturing.
The UN's top climate official said firm US targets for reducing emissions were necessary for a deal.
"I believe the US can commit to a number [at the summit] in Copenhagen," said Yvo de Boer, executive secretary of the UN climate convention (UNFCCC).
"There was a number in Mr Obama's campaign, there are numbers in the legislation [going through Congress] - so I believe it's perfectly possible for the US to sign up to a specific pledge."
US legislation setting caps on emissions is under discussion in the Senate, and unlikely to be finalised this year.
Quizzed by reporters as to whether the US would bring an emissions target to Copenhagen, its delegation chief here, Jonathan Pershing, said it was possible but a decision had not yet been taken.
"The science demands urgent action, and the US is committed to our fair share," he said.
"Developed countries including the US must put numbers forward; developing countries except the least developed countries must [also] make commitments."
The Copenhagen summit is supposed to conclude the two-year process that began at the Bali summit two years ago of formulating a treaty to supplant the Kyoto Protocol.
Demonstrators in Copenhagen called for carbon emissions to be cut
The absence of a US target is the most glaring hole in the draft treaty's complex tapestry that weaves together concerns over reducing emissions from rich countries, reducing the rate of emissions growth in more prosperous developing nations, and providing financial assistance to the developing world.
Developing countries have regularly accused the West - and the US in particular - of failing to live up to its international obligations.
"It seems that developed countries have been negotiating with their economic interests at heart rather than [the world's] environmental interests," said Angelina Navarro Llanos, head of the Bolivian delegation.
But Mr Pershing insisted that developing countries were also to blame for the apparent impasse that earlier in the week brought a walkout from African delegations.
"Developing countries want a legal deal that applies to us but not to them," he said.
The rancour evident during the week has led some observers to conclude there is no chance of achieving a legally binding deal in Copenhagen.
Many people close to the talks - including Mr de Boer - have played down the chances of tying up a full treaty this year, and there are suggestions it could take a further full year to conclude.
But others said the picture was not as dark as it seemed, and that last-minute policy revelations and compromises were still likely.
"There's a fair degree of political posturing," said Kumi Naidoo, chair of the activist coalition tcktcktck.
"[But] some of the negotiators from developed countries have come to the conclusion that they're not going to deliver what was promised in Bali, and there's a lowering of expectations going on."
Most developed countries set out targets for reducing their emissions well before this meeting convened - and they add up to a lot less than the 40% (from 1990 levels by 2020) that developing nations are demanding.
On the face of it, this is an unbridgeable divide; but other delegates suggested the G77/China bloc of developing countries would probably settle for about 30% provided other elements of a package - such as plentiful finance - were also on offer.
All countries have been urged to take part and negotiate
Mr de Boer said the Danish conference hosts had a list of 40 heads of state or government intending to attend - though it is not known whether US President Barack Obama is among them.
Some delegates suggested only political engagement at this level could turn the current situation into a new binding agreement.
Most of the negotiating here has taken place in small groups tackling specific issues; and it is clear that progress has been uneven.
Negotiations towards an agreement on Reducing Emissions from Deforestation and forest Degradation (Redd) were progressing well, delegates said, with a good chance of agreeing something in Copenhagen that could see money begin to flow into forest protection next year.
Mr de Boer cited the transfer of clean technology from the industrialised to the developing world as another area where progress has been made.
But other areas of negotiation - notably on raising money to help poor countries adapt to climate impacts, and on commitments to reduce emissions - have clearly been more difficult.
There is still disagreement too over the legal form of a new agreement at or after Copenhagen - whether it should take the form of an extension to the Kyoto Protocol, which Mr de Boer noted was the only legal agreement in existence that had curbed carbon emissions, or another legal entity.
Elements of the draft treaty remain in the form of "non-papers", delegates said - documents that do not carry the weight of a formal negotiating text.
Delegates will convene in the Danish capital on 7 December for two weeks of negotiations; although the introduction of key elements is likely to be withheld until the final few days.