Next month's summit on climate change in Copenhagen must set a deadline for a legally binding document, Denmark's climate minister said today.

Connie Hedegaard said it is very important to set the deadline "as soon as possible" in the text to be agreed upon in the Danish capital. She spoke at the start of a two-day closed meeting of climate negotiators from nearly 40 countries who are preparing for the Copenhagen UN summit, which starts on 7 December.

The head of the UN climate change secretariat, Yvo de Boer, said participants at the Copenhagen meeting must come up with "a series of clear decisions" in order to have a treaty within six months of the conference ending. It would be designed to replace the 1997 Kyoto protocol, which expires in 2012.

Yesterday, Barack Obama supported plans to delay a legally binding deal until next year at the earliest, pushing for next month's meeting to become a first-stage series of commitments rather than an all-encompassing protocol. Michael Froman, US deputy national security adviser for economic affairs, said: "There was a realistic assessment ... by the leaders that it was unrealistic to expect a full internationally legally binding agreement to be negotiated between now and when Copenhagen starts in 22 days."

Germany also said today that Chancellor Angela Merkel plans to attend the summit on its final two days, after annoucements last week by leaders including Gordon Brown and the Australian prime minister, Kevin Rudd, that they would be attending. Merkel's spokesman, Christoph Steegmans, said the chancellor does not expect the meeting to produce a legally binding accord to cut greenhouse gas emissions, but that she 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.

The Danish prime minister, Lars Lokke Rasmussen, is urging world leaders to attend the conference, which has been billed by many as a last chance to halt dangerous climate change. Obama has yet to confirm he will be going, though this month he said he would attend if countries were on "the brink of a meaningful agreement".