Clinton, the first US secretary of state to take part in the annual Pacific Islands Forum, pledged $25 million in new assistance to help the low-lying nations of the region plan and adapt to expected rising water levels.

She said that the United States stood behind President Barack Obama's promise in 2009 ahead of a UN conference in Copenhagen to curb greenhouse gas emissions blamed for climate change by 17 percent by 2020 from 2005 levels.

"As you know, in part because of the economy, US emissions are the lowest that they've been in 20 years," Clinton told a joint news conference with New Zealand Prime Minister John Key in the Cook Islands.

"But, look, we know we have more to do. And we have made a commitment -- we're going to stick with our commitment. I hope that we'll be able to go beyond those commitments in the future," Clinton said.

Proposals backed by Clinton's Democratic Party to order mandatory curbs on emissions have failed in the US Congress, where rival Republicans question the science behind climate change and say action is too costly.

US emissions nonetheless fell to their lowest level since 1992 in the first three months this year in part because lower prices for natural gas have reduced use on coal, the dirtiest major form of energy.

UN-led negotiations on climate change have been torn by division between wealthy and developing nations, despite pleas from small island nations for more aggressive action.

Clinton said that she understood the "great passion" of South Pacific nations.

"This is real -- I will underscore that," she said of climate change.