Large amounts of a powerful greenhouse gas are bubbling up from a long-frozen seabed north of Siberia, raising fears of far bigger leaks that could stoke global warming, scientists said.

It was unclear, however, if the Arctic emissions of methane gas were new or had been going on unnoticed for centuries - since before the Industrial Revolution of the 18th century led to wide use of fossil fuels that are blamed for climate change.

The study said about eight million tonnes of methane a year, equivalent to the annual total previously estimated from all of the world's oceans, were seeping from vast stores long trapped under permafrost below the seabed north of Russia.

"Subsea permafrost is losing its ability to be an impermeable cap," Natalia Shakhova, a scientist at the University of Fairbanks, Alaska, said in a statement. She co-led the study published in yesterday's edition of the journal Science.

The experts measured levels of methane, a gas that can be released by rotting vegetation, in water and air at 5,000 sites on the East Siberian Arctic Shelf from 2003-2008. In some places, methane was bubbling up from the seabed.

Previously, the sea floor had been considered an impermeable barrier sealing methane, Shakhova said. Current methane concentrations in the Arctic are the highest in 400,000 years.

"No one can answer this question," she said of whether the venting was caused by global warming or by natural factors. But a projected rise in temperatures could quicken the thaw.

"It's good that these emissions are documented. But you cannot say they're increasing," said Martin Heimann, an expert at the Max Planck Institute for Biogeochemistry in Germany who wrote a separate article on methane in Science.

"These leaks could have been occurring all the time" since the last Ice Age 10,000 years ago, he said. He wrote that the release of eight million tonnes of methane a year was "negligible" compared to global emissions of about 440 million tonnes.

Dr Shakhova's study said there was an "urgent need" to monitor the region for possible future changes since permafrost traps vast amounts of methane, the second most common greenhouse gas from human activities after carbon dioxide.