The United Nations used the occasion of World Environment Day on Thursday to demand action on climate change for the protection of small island states threatened by rising seas, drought and floods.

"We have very, very alarming news from science," UN climate chief Christiana Figueres told reporters in Bonn at a new round of talks on global warming.

"Greenhouse gas concentrations continue to rise, with the most threatening impact happening on small island states."

UN Secretary General Ban Ki-moon said the world's 52 small island states contributed less than one percent of greenhouse gas emissions annually, yet found themselves "on the front lines" of the consequences.

"I urge everyone to think about the plight of small island developing states and to take inspiration from their efforts to address climate change, strengthen resilience and work for a sustainable future," Ban said in a press statement.

"Raise your voice, not the sea level. Planet Earth is our shared island. Let us join forces to protect it."

The 12-day Bonn talks, which began on Wednesday, seek to overcome some of the many obstacles towards a post-2020 pact to roll back heat-trapping fossil-fuel gases.

The deal, aimed at limiting warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels, is due to be finalised in Paris at the end of 2015.

But the talks are hampered by labyrinthine complexity and squabbling over which countries should shoulder most of the bill.

The Bonn session is a stepping stone to a special UN climate summit in September and the annual decision-making conference in December in Lima under the UN Framework Convention on Climate Change (UNFCCC) banner.

- Climate change already underway -

Ministers from three small island countries -- Grenada, the Maldives and the Marshall Islands -- said their nations were already being hit by a mutating climate, notably by salt intrusion into coastal fields and declining fish catches.

The Maldives, as a result, is having to ship drinking water to many inhabitants, said its environment minister, Abdullahi Majeed.

"This costs us millions of dollars," he said.

"We don't have much time to drag our feet and debate the issues. The science is very clear and we all know what to do."

Majeed's counterpart Roland Bhola of Grenada said many of the Caribbean island's inhabitants lived on the coastline and relied on the resources of the sea for their survival.

"Our fishermen are reporting less and less catches where there was once a thriving trade," he said. "The salinity of the water is affecting the very survival of some of our coastal farmers."

In a report issued Thursday, the UN Environment Programme (UNEP) said climate change could inflict "trillions of dollars" in annual costs on small island states.

Destruction of beaches would devastate tourism, and loss of corals would wreck fishermen's livelihoods -- driving up dependence on imported food.

Some island states in the western Pacific saw sea levels rise at a rate of 12 millimetres (0.5 inches) per year between 1993 and 2009, or about four times the global average, it said.

In the Caribbean, warmer waters have already bleached up to 100 percent of coral reefs in some insular states.

"Climate threats are projected to push the proportion of reefs at risk in the Caribbean to 90 percent by 2030 and up to 100 percent by 2050," according to the report.

If the world's 34 million hectares (84 million acres) of coral reefs were wiped out, that would inflict annual losses of $11.9 trillion (8.8 trillion euros), UNEP estimated.