COPENHAGEN - China urged President Barack Obama to increase a U.S. offer to cut carbon emissions but its top climate envoy indicated willingness on Wednesday to compromise at a U.N. conference in Copenhagen.

Xie Zhenhua said that China wanted to play a constructive role at the December 7-18 climate talks, where a successful outcome largely depends on agreement between the United States and China which together emit 40 percent of global greenhouse gases.

I do hope that President Obama can bring a concrete contribution to Copenhagen, Xie told Reuters.

When asked whether that meant something additional to what Obama has already proposed, a 3 percent cut on 1990 levels by 2020, Xie said: Yes.

Xie also said that China could accept a target to halve global emissions by 2050 if developed nations stepped up their emissions cutting targets by 2020 and agreed to financial help for the developing world to fight climate change.

We do not deny the importance of a long-term target but I think a mid-term target is more important. We need to solve the immediate problem.

If the demands of developing countries can be satisfied I think we can discuss an emissions target, to halve global emissions by 2050.

The deputy chairman of the powerful economic planning superministry, the National Development and Reform Commission (NDRC), told Reuters he wanted rich countries to cut their emissions by 25-40 percent below 1990 levels by 2020.

It is our hope that the emissions cuts of developed countries can fall into the range of 25-40 percent (below 1990 levels. Earlier this year, at some previous rounds of U.N. talks, China had insisted on a cut of at least 40 percent.

Xie said that he preferred a final, legally binding agreement at the meeting in Copenhagen, but if that were not possible a deadline to wrap up a full treaty by June would be very good. He rejected a U.N. proposal for fast-track funding of $10 billion a year from 2010-2012 as not enough.

(Writing by Gerard Wynn; Editing by Dominic Evans)