Energy giant Total said it will send a team of experts by helicopter to a stricken North Sea gas platform on Wednesday or Thursday to assess how to stop a potentially explosive gas leak.

The French firm evacuated the Elgin rig off the Scottish coast on March 25 because of a gas leak which the company says is costing it $2.5 million (1.87 million euros) a day.

A Total spokesman told AFP on Tuesday that there would be a "helicopter flight tomorrow or the day after tomorrow, depending on the weather conditions, to put a first team in place."

The spokesman said the development came after Total agreed conditions with Britain's Health and Safety Executive to "regain safe access to the platform" and that they were aiming for Wednesday afternoon or Thursday morning.

Scotland experienced blizzards on Tuesday just days after basking in record temperatures for the time of year, with deep snow and temperatures of minus 0.5 degrees Celsius (33 degrees Fahrenheit).

After a flare on the rig extinguished itself, lowering the risk of an explosion from the estimated 200,000 cubic metres of highly flammable gas leaking each day, Total has been focusing on efforts to stop the leak.

It is moving two rigs from elsewhere in the North Sea to drill two relief wells, in parallel with an operation to pump "heavy mud" at high pressure into the stricken well.

But a team must first be airlifted on to the platform to make a preliminary assessment of how best to stop the leak, it said.

Total, which has described the leak as its worst problem in the North Sea in a decade, said on Monday that the team plugging the leak would include outside experts from Texas-based firm Wild Well Control.

The firm was among those that worked to stem the massive oil spill following an explosion at BP's Deepwater Horizon rig in the Gulf of Mexico in 2010.

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.

Greenpeace activists sailed to the area on Monday and reported an oily sheen stretching over the sea's surface for several miles, but both Total and British authorities say there has been no significant effect on marine life.

Total has readied a Hercules military transport plane carrying dispersant that could be sprayed on the sheen, but it said the substance was gas condensate that would probably evaporate by itself.

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 damage to its reputation and finances caused by the Deepwater Horizon spill.