UN climate change chief Christiana Figueres on Tuesday warned time was running out for meaningful action on global warming, citing the plight of low-lying Pacific nations facing ever rising seas.

Figueres, in Samoa for a UN conference on small island states, said the impact of climate change was greatest on Pacific nations, even though they had contributed little to the problem.

"Climate change is the greatest threat these islands face and they are recognised as the bellwether of global efforts to address this issue," she told AFP.

"Unless the world acts on climate change in a timely way, they are going to be the hardest hit."

Figueres said rising seas not only eroded the coastlines of island states, they also spoiled water supplies when they entered the water table and swamped agricultural land, rendering it barren.

Warming also meant more cyclones and storms battered the islands, while planning was underway for a worst-case scenario where populations of climate change refugees would have to be relocated from their homelands.

"Kiribati (which has purchased land in neighbouring Fiji) is probably the most famous, but countries as large as Papua New Guinea are already starting to identify which are their most threatened populations," she said.

"These are extreme measures that these islands are having to look at. Of course they, and the rest of the world, want migration of populations out of the islands to be kept at a minimum."

Figueres said the situation facing island nations underlined the need for progress in the quest to seal a global pact on greenhouse gas emissions by the end of 2015.

The UN wants to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels, which scientists say is the minimum needed to stabilise the climate.

"The science tells us that we have to stay under two degrees temperature-wise and that the door is closing quickly," she said. "It's still possible for us to stay under two degrees but we have to do it."

Island leaders have become increasingly vocal on the issue in the face of global inaction, with Seychelles President James Michel telling the Samoa conference that the interests of big business have dominated the debate for too long.

"It is time that we recognise climate change for what it is -- a collective crime against humanity," he said. "Climate change... is robbing island nations of their right to exist. We must save our future together."

Marshall Islands President Christopher Loeak said island nations had to get across a positive message about what needs to be done at a UN summit in New York this month, which will be followed by an attempt in Paris next year to forge a new climate deal.

"The time for finger-pointing is long past... instead, we must recognise that there is no more powerful form of leadership that leadership by example," he said.