EU Environment Commissioner Stavros Dimas told Reuters in an interview that the EU must stick to what it believes in -- legally binding emissions cuts -- despite calls from some corners this week for a less rigid stance.

"I think what we should do is try to get on everybody on board with mandatory reductions," said Dimas, a Greek lawyer who has held this post for less than a month.

US delegation head Paula Dobriansky ruled out mandatory caps and reiterated it is too soon to talk about post-2012.

"Our policy is that we do not support mandatory targets or timetables," Dobriansky, Undersecretary for Global Affairs at the State Department, said in an interview.

Under Kyoto, industrialized countries together must reduce man-made emissions by five percent by 2012 versus 1990 levels and some have deeper cuts than others. Most consider it a first, small step to stop global warming.

The United States, the biggest polluter in the world with 25 percent of heat-trapping greenhouse gas emissions, chafes at the idea of a mandatory caps, which it says thwart growth. That was the main reason President George W. Bush pulled out of Kyoto in 2001.

While few of the 6,000 people at the convention expected the United States to change its mind on Kyoto, many were surprised at its hardline stance during the last 10 days.

"See a few more summers in Texas where the roads are melting, or the drought wipes out the farmers and the ranchers ... then you might start to see some rethinking," said US Rep. Dennis Kucinich, an Ohio democrat and supporter of Kyoto.

'FRUSTRATED, DISAPPOINTED'

Italy this week suggested that maybe it was time for negotiators to discuss voluntary targets for after 2012 as a way to entice the United States and fast-growing developing giants China and India, now among the top five polluters.

But Dimas is doubtful of this approach, particularly for industrialized nations.

"Voluntary reductions have not given results up to now. So we need mandatory reduction targets," he said.

Dobriansky said she preferred a "bottom-up" commitment in which countries propose their own goals.

Bush has a 10-year domestic program anchored on research and technology to cut "greenhouse gas intensity" 18 percent by 2012. UN data show US emissions are up 13 percent since 1990 and Dobriansky could not say when they would go below 1990 levels.

Washington's stonewalling has many negotiators and activists worried the Buenos Aires conference may end on Friday without any progress in talks. They point to reports that show the world in the midst of some of its hottest weather and biggest natural disaster destruction.

Host country Argentina and the EU are keen on some seminars for informal talks next year to keep climate change talks rolling. But the United States says it won't back them if they look to lead into post-2012 negotiations.

"They are trying to hold back even informal discussions," said Jennifer Morgan, director of climate change at leading environmental group WWF.

Even Dimas shows signs of losing patience with the slow pace of Buenos Aires, a conference that started on a high note after Russia's ratification of Kyoto last month which allows the treaty to go into force Feb. 16.

"I feel a bit frustrated and disappointed that we have such long discussions," Dimas said.