(CNN) — U.S. Secretary of State Hillary Clinton arrived in Copenhagen early Thursday morning amid concerns that time is running out at the climate change summit for world leaders to agree a deal to combat global warming.

Yvo de Boer, the U.N.’s top climate official, admitted Wednesday evening that negotiations had unexpectedly stalled and said that the next 24 hours would be crucial.

The conference’s Danish hosts had been expected Wednesday to table a text intended to establish a basis for further negotiations. But de Boer said he did not know if the Danish text had been tabled.

“The cable car has made an unexpected stop,” De Boer told journalists. On Monday he had said that the “cable car” was halfway up the mountain and that the rest of the ride would be “fast, smooth and relaxing.”

Clinton is among dozens of senior-level figures joining the negotiations in the final days in an effort to push the summit towards a global deal limiting carbon emissions to replace the Kyoto Protocol. U.S. President Barack Obama is expected to join the talks Friday.

“The Secretary and the President decided that she could play a useful role in helping close gaps in our climate talks there by traveling to Copenhagen and personally participating,” a spokesman for the U.S. State Department said.

Chinese Premier Wen Jiabao was among world leaders to arrive at the summit Wednesday. He told reporters aboard his flight to Denmark that he intended to assert China’s “sincerity and determination” to work with the international community to tackle climate change,” according to the official Chinese government Web site.

“I hope the meeting, with joint efforts made by various parties, will yield fair, reasonable, balanced and achievable results,” he said.

Japan became the latest country to pledge climate aid to developing nations Wednesday, offering $15 billion by 2012 to help vulnerable states mitigate against the impact of global warming. Earlier in the week the European Union pledged $9.4 billion for the same purpose.

Climate change activists attempted to disrupt the summit Wednesday, resulting in around 250 arrests. Protesters had hoped to get inside the Bella Center, where the talks are being held, to set up a “people’s assembly” but police used pepper spray and dogs to contain the demonstrations.

A spokesman for the Copenhagen police told CNN that the majority of arrests had taken place outside the center but there had been no serious injuries.

While the protests were going on outside, inside it was being announced that Danish minister Connie Hedegaard had resigned as president of the U.N. climate change summit.

“The resignation is essentially procedural,” CNN’s Phil Black said, “and she’ll be replaced by the Danish prime minister. It’s a reflection of the fact that the talks are now at the high-level phase, and it was deemed appropriate that the PM should now take over.”