Differences between countries are threatening UN climate talks in Tianjin, where developing nations complained that rich countries have been slow to deliver financial aid to help combat climate change promised last year at Copenhagen.

World leaders had agreed to a "fast-start" climate fund of US$30 billion for forestry and other efforts in developing nations – which face the worst consequences of climate change – to tackle climate change from 2010 to 2012.

So far, wealthy nations have pledged almost the entire amount, though only a small percentage has been dispersed. Many climate justice advocates also say it is unclear how the money is being transferred.

"Although some of this amount has been identified, it's still not very clear that how the money is moving through the mechanism," said Stanley So of Oxfam Hong Kong. "They (developed countries) have to show us how those pledged funds are used in urgent-need, developing countries in a transparent manner."

Final negotiations in Tianjin also will likely see developed nations channeling US$100 billion a year into climate funds by 2020. But any aid from developed nations will come with strings attached, if they get their way.

"Developed countries, especially the US government, want to see the developing nations, particularly China, agree to accept the measurable, reportable and verifiable process, meaning that any actions taken by governments to cut carbon have to be transparent to the international community," said David Waskow of Oxfam America.

The measurable, reportable and verifiable (MRV) mechanisms were established under the Copenhagen accord last December as a way to check the progress of developed countries' emissions reduction.

The U.S. is unlikely to further assist developing countries unless they accept the MRV mechanisms, Waskow said.

China is open to the requirement but still stresses that developed countries should fulfill their commitments first. "China wouldn't object to the MRV system, but developed countries should follow their commitment in the Kyoto Protocol first, which includes climate financing, technology transfer and reporting," said Xie Zhenhua, China's chief climate negotiator, on Monday at a press conference.

Waskow remains optimistic that a long-term financing plan will receive US support. Several key members of Congress wrote a letter to Secretary of State Hillary Clinton on Tuesday expressing their strong support for the establishment of a global climate change fund.

"Not all members of Congress disagree with taking more efforts on climate change," Waskow said.

The UN climate talks in Tianjin are the last round of negotiations before the UN's annual summit, which this year will be in Cancun, Mexico.

"Setting up a climate fund that gives a voice to those facing the harshest climate impacts is one of the things that can get the talks moving this year," So said.