A research vessel owned by environmental group Greenpeace arrived on Monday in the vicinity of a North Sea platform that has been leaking highly flammable gas for more than a week.

French energy giant Total, which operates the stricken Elgin rig 150 miles (240 kilometres) off Aberdeen in eastern Scotland, has insisted the leak does not pose a significant threat to the environment.

But the Greenpeace ship Koenigin Juliana ship set sale from Germany on Saturday with activists saying they wanted to assess the environmental impact of the accident for themselves.

"We're now on location," said Greenpeace logistics officer Michael Meyer from the edge of a two nautical miles (3.7 kilometres) around the platform.

"We've done some air tests and we're now taking water samples," Meyer said.

An AFP photographer onboard the ship said there was a faint smell of gas in the air and a multi-coloured sheen was visible on the choppy waters.

A sheen of gas condensate stretching several miles spread over the water around the platform in the days following the accident at the rig, which continues to leak an estimated 200,000 cubic metres of flammable gas each day.

Total said it expects the sheen to evaporate by itself, but a Hercules military transport plane carrying dispersant is on standby.

The company, which has seen an estimated eight billion euros ($10 billion) wiped off its stock value since the leak was discovered on March 25, is awaiting British regulators' advice on whether it is safe to approach the rig.

Total has assembled a team of experts in Aberdeen and is preparing to drill two relief wells to stop the gas leak, in parallel with an operation to pump so-called "heavy mud" at high pressure into the stricken well.

The last of Elgin's 238 crew were evacuated on Monday, while Total's Anglo-Dutch rival Shell has also been forced to halt output at its Shearwater platform and Noble Hans Deul rig, four miles away, because of safety concerns.