Negotiators meeting for a final session before a worldwide conference on climate change were urged on Monday to craft simple, clear options for politicians facing next month's haggle in Copenhagen.

"The clock has almost ticked down to zero and, as always, time will fly," the head of the UN's climate convention, Yvo de Boer, warned the 192-nation forum, meeting in Barcelona until Friday.

"These last five days are critical on the road to success to Copenhagen. They need to be used wisely."

Senior officials have been meeting over the last two years under a "road map" leading to the December 7-18 showdown in Copenhagen.

If all goes well, it will craft a new pact on climate change beyond 2012, when current pledges expire under the Kyoto Protocol.

But the negotiations are mired in discord.

Rich countries and poor countries are squabbling over how to apportion curbs in carbon emissions, finance a switch to lower-pollution technology and shore up defences against climate change.

Successive rounds have given birth to a baffling, bloated draft text, thick with brackets denoting discord.

Connie Hedegaard, Danish minister for climate and energy, who will chair the Copenhagen talks, urged negotiators to "cut the plenaries short and go directly to smaller negotiating groups and informals" to spur progress.

"Your job now is to create clear options for politicians, clear options across the building blocks, in order for ministers to decide in Copenhagen," she said.

In Moscow, visiting Danish Prime Minister Lars Loekke Rasmussen said he expected the Copenhagen finale to yield a political accord that would be a stepping stone to a treaty.

"I expect a politically binding agreement that will take effect right after the signature. This agreement will be the basis for a legally binding agreement," Rasmussen said.

Green groups took aim at Washington, protesting that despite policy shifts under President Barack Obama, the US was still failing to pull its weight.

A climate bill is wending its way through Congress, but in the international arena the United States has yet to put forward detailed proposals.

It is also pushing the idea that commitments made in the future treaty be "legally binding" at national level rather than policed by tough international compliance mechanisms.

"To earn his Nobel, President Obama must put an end to this isolationist nonsense and commit to acting cooperatively to solve the climate crisis," said Kate Horner of the US branch of Friends of the Earth.

Russian Prime Minister Vladimir Putin said on Monday his country would commit to a post-2012 deal provided the pact was signed by all countries and Russia's enormous forests were "taken into account."

Russia and other countries demanded big concessions on forestry in 2001 when Kyoto's complex rulebook was being negotiated.

They argued that forests are a bulwark against global warming as trees absorb carbon dioxide (CO2) -- the principal greenhouse gas -- under the natural process of photosynthesis.

The issue of how much forested land should be offset against emissions targets by rich countries turned out to be a major stumbling block in those talks.

Many green activists say the forestry rules are a potential loophole, enabling polluting countries to statistically write off their emissions yet not reduce them in real terms.

Greenpeace militants, meanwhile, scaled Barcelona's landmark Sagrada Familia church to display a protest banner, while campaigners with Oxfam International tore up cheques, symbolising what they called the scornful attitude of rich countries towards the poor.

Scientists are clamouring for early, drastic measures to curb CO2 and other greenhouse gases emitted by fossil fuels and deforestation.

On present trends, these heat-trapping gases could cause catastrophic damage to the world's climate system, leading to hunger, drought, rising oceans and melting snowcaps just decades from now, they say.