COPENHAGEN (bdnews24.com/Reuters) - Environment ministers tried to overcome rifts between rich and developing nations in Copenhagen on Sunday just days before a deadline for reaching a global pact on tackling climate change. Yvo de Boer, head of the UN Climate Change Secretariat, highlighting a spat between top greenhouse gas emitters China and the United States, said he hoped all nations would seek to raise their offers in the talks. "China is calling on the United States to do more. The United States is calling on China to do more. I hope that in the coming days everyone will call on everyone to do more," he said. The ministers were holding informal talks during a one-day break in the December 7-18 meeting involving 190 nations, which will culminate in a summit of world leaders on Thursday and Friday including US President Barack Obama. "There are still many challenges. There are still many unsolved problems," Danish Minister Connie Hedegaard told reporters. "But as ministers start to arrive there is also the political will." The talks bring together representatives from rich and poor nations who have been arguing over who is responsible for emissions cuts, how deep they should be, and who should stump up cash to pay for them. Countries like China and India say the industrialized world must make sharper reductions in greenhouse gas output and provide the poor with more cash to fund a shift to greener growth and adapt to a warmer world. "An agreement is certainly possible. If all of us trust each other and if we have the courage and conviction, we can still come to a fair, equitable deal in Copenhagen," Indian Environment Minister Jairam Ramesh said, heading into Sunday's sessions. Richer countries say the developing world's carbon emissions are growing so fast they must sign up for curbs to prevent dangerous levels of warming. China has said it wants to wrap up a firm deal before Premier Wen Jiabao joins other world leaders at the summit. "My understanding is that the leaders are coming to celebrate the good outcome of the talks," senior Chinese envoy Su Wei said on Saturday. DEMONSTRATORS RELEASED On Sunday, South African Nobel Peace Prize winner Archbishop Desmond Tutu handed over to the UN's de Boer tens of thousands of signatures from around the world calling for climate action. An afternoon church service was also planned at Copenhagen's Cathedral, with a sermon by Archbishop of Canterbury Rowan Williams and attended by Danish royalty, followed by a "bell ringing for the climate" in churches around the world. Police have released all but 13 of nearly 1,000 people detained after a march on Saturday, a police spokesman said. The march by tens of thousands of people was largely peaceful but violence erupted toward evening when demonstrators smashed windows and set fire to cars. Some of those detained said they were unfairly held and badly treated by police. "They arrested us for no reason. We were all peaceful," said Hana Nelson, aged 24, a student from Halifax, Canada, who was released without charges.