French energy giant Total was awaiting advice from British regulators on Sunday on whether it was safe to approach a North Sea platform leaking flammable gas for a week.

Britain's Health and Safety Executive (HSE) "has received a risk assesment from Total and the process of thorough examination of the documentation is taking place," an HSE spokesman told AFP.

"HSE will not speculate on how long this process will take," he said, stressing that Total did not legally need the regulator's permission to approach the Elgin rig, 150 miles (240 kilometres) from Aberdeen in eastern Scotland.

Total's UK communications manager Andrew Hogg said the firm planned to send a team of experts to the rig, still leaking an estimated 200,000 cubic metres of highly flammable gas each day, "in the next couple of days".

Hogg would not comment on whether the firm would send a team to the rig without a green light from the HSE, but he told AFP: "There is no way that we would put our people in jeopardy."

"We want to check out the well-head and ensure we know what equipment we need to tackle the leak," he said, adding that the team would include 'well control' specialists as well as staff familiar with the layout of the rig.

Four fire ships were on standby on Sunday on the edge of a two nautical mile wide exclusion zone around the platform, while Total said it had a team of experts and engineers at a makeshift crisis centre in Aberdeen.

The rig's 238 crew were evacuated after the leak was discovered last Sunday. Safety concerns have forced Total's Anglo-Dutch rival Shell to halt output at its Shearwater platform and Noble Hans Deul rig four miles away.

Total said on Saturday that a flare on the rig, which had threatened to come into contact with the low-lying gas cloud and cause an explosion, had gone out -- but that there was still a risk of ignition.

"There is still a gas escape, and clearly escaping gas is always at risk of ignition and explosion," Hogg told AFP.

Total has seen around eight billion euros ($10 billion) wiped off its stock value since the last of Elgin's crew were evacuated on Monday.

The company is preparing to sink two relief wells to stop the gas leak, in parallel with an operation to pump so-called "heavy mud" into the stricken well at high pressure.

Hogg said the first of two rigs being moved from elsewhere in the North Sea, to help drill the relief wells, was expected to arrive this week.

A sheen of gas condensate stretching several miles spread over the water around the platform in the days after the leak.

Total has insisted that the sheen is evaporating and does not pose a significant threat to the environment, but a Hercules military transport plane carrying dispersant is on standby.

A research vessel owned by the environmental group Greenpeace set sail from Germany towards the rig on Saturday, to make its own assessment.

Greenpeace said experts on board the ship, due to arrive at the edge of the exclusion zone on Monday, would take samples to measure air, water and soil pollution.

The last major accident in the North Sea was in 1988, when the Piper Alpha oil platform operated by the US-based Occidental Petroleum exploded, killing 167 people.

Total's British rival BP is still recovering from the damage to its reputation and finances caused by an explosion at its Deepwater Horizon oil rig in the Gulf of Mexico in 2010.