COPENHAGEN (AP) — With a global warming pact out of reach this year, a key climate summit in Copenhagen next month must set a deadline for a legally binding treaty, Denmark’s climate minister said today.

Ambitions for the U.N. talks in Copenhagen have been scaled back in recent weeks. President Barack Obama and other world leaders said Sunday it was unrealistic to expect them to complete a full treaty to cut greenhouse gas emissions, but that an agreement on all the political elements of a deal was essential.

At a meeting of climate negotiators preparing for the Dec. 7-18 U.N. summit, Danish Climate Minister Connie Hedegaard said leaders should be focused on setting a deadline for the final text — possibly at talks set for December 2010 in Mexico City.

“I would like to see it as soon as possible, maybe a realistic deadline would be Mexico, but it depends on how close the parties will come to an understanding on crunch issues,” she said.

Arriving for the start of the two-day meeting, the head of the U.N. climate change secretariat, Yvo de Boer, said he hoped a final accord could be reached by next June.

He said participants at the Copenhagen conference must come up with “a series of clear decisions” to clear the way for the full treaty. It would succeed the 1997 Kyoto Protocol, which expires in 2012.

“If governments sign up to a series of decisions in Copenhagen in three weeks time, then six months later if we turn that into a treaty text, (they) will be politically bound by these decisions,” de Boer said. “If these kinds of decisions are taken, it would be no problem to turn it into a full treaty in six months’ time.”

The Danish hosts of the Copenhagen conference have proposed to make the goal a matter of crafting a “politically binding” agreement, in hopes of rescuing some future for the struggling process.

U.S. climate envoy Todd Stern said that could include a deadline for a legally binding treaty.

“I expect there probably will be but we’re open for discussions. But something like that probably makes sense, we’ll be discussing that,” he said.

Denmark on Thursday sent invitations to world leaders in 191 countries to attend the climate summit. At least 40 leaders have said they plan to attend the conference, including British Prime Minister Gordon Brown, French President Nicolas Sarkozy and Dutch Prime Minister Jan Peter Balkenende.

Germany announced today that Chancellor Angela Merkel also plans to attend.

Merkel’s spokesman, Christoph Steegmans, said Merkel does not expect the meeting to produce a legally binding accord, but that the chancellor expects “an important step toward a treaty” to be made.

Merkel was instrumental in securing the Kyoto Protocol, which was approved while she served as Germany’s environment minister.

Obama has said he would consider going to Copenhagen. Obama and nearly two dozen fellow leaders from Asia and Europe agreed Sunday that the Copenhagen talks will be a way station — not the end point — in the difficult and so-far elusive search for a new worldwide treaty to tackle global warming.

A major bill dealing with energy and climate in the U.S., a domestic priority of Obama’s, is bogged down in the U.S. Senate with scant hope it would be completed by next month, giving the American president little to show in Copenhagen.

— AP writer Melissa Eddy contributed to this story from Berlin.