French energy giant Total said Monday a gas leak in the North Sea platform is costing it $2.5 million (1.87 million euros) a day, as Greenpeace said the rig was surrounded by an oily sheen.

Total's financial director Patrick de la Chevardiere told journalists in Paris that in addition to spending $1 million a day on trying to fix the leaking Elgin rig, the company was losing $1.5 million in daily revenue.

It has moved two drilling platforms into the area off the Scottish coast and is preparing to sink two relief wells to stop the leak, in parallel with an operation to pump "heavy mud" at high pressure into the stricken well.

Total says it is also planning to land contractors on the rig in coming days but is first considering advice from British regulators.

A Greenpeace ship, the Koenigin Juliana, spent Monday about three nautical miles from the platform, at the edge of an exclusion zone around the abandoned platform which was evacuated of its 238 crew on March 25.

Christian Bussau, a marine expert from Greenpeace on board the ship, said a multi-coloured sheen around the platform was spreading and the group took air and water samples.

He said he believed the substance to be oil, though he admitted Greenpeace would not be able to analyse its samples until the ship returned to its base in Germany. It set sail for home on Monday evening.

"This is a really big accident," Bussau said. "Total must immediately start to close the leak, or the pollution won't stop."

Total says the sheen is caused by gas condensate, a gas-derived liquid hydrocarbon, and not oil.

Total has readied a Hercules military transport plane carrying dispersant that could be sprayed on the sheen, but said it did not expect it would be necessary to deploy it.

"The light condensate poses no significant threat to seabirds or other wildlife," a Total spokeswoman told AFP.

The costs to Total from the leak are relatively low, compared to previous high-profile disasters, de la Chevardiere told investors, and insisted the firm's financial plans were unchanged.

"There's no reason to change our programme in terms of investment ... nor our undertaking to pay a competitive share dividend," he said.

The company has said it is assembling a crew to go on to the platform "in the next couple of days".

Specialist offshore safety inspectors from Britain's Health and Safety Executive (HSE) met with Total representatives in Aberdeen on Monday to give them "further advice" on the plans, the HSE said.

"Total is in the process of considering the responses offered by HSE and continues to plan its operation to board the installation in light of that," it said.

Total said the crew would include outside experts from Texas-based firm Wild Well Control. They 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.

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.