France's Total sent fire-fighting ships close to the scene of a gas leak from its North Sea Elgin platform on Thursday as a large gas cloud led to fears of an explosion.

The company said the gas originated thousands of meters below the seabed, which engineers said might mean that a relief well - one possible option to stop the leak - could take months to drill.

Total has not yet found a way to stop the gas leak.

"The bad news is that the leak is continuing and that it reduces the possibility it could be plugged by sand or other material," Frederic Hauge, head of leading Norwegian green group Bellona, which has a team of oil experts monitoring Total's response, told Reuters.

"The good news is that the flow rate of gas coming to the surface is not increasing."

A team of international engineers assembled by the embattled French oil company are drawing up plans to tackle the leak and prevent the flare from coming into contact with the gas cloud, the spokeswoman said.

The platform is currently off limits to the engineers, however, given the toxic and explosive plumes pumping out of the wellhead.

A flare needed to relieve pressure in the platform by purging excess gas has continued to burn less than 100 meters from the leak, and engineers said changes in wind and weather could lead to an explosion.

"The wind is pushing the gas cloud in the opposite direction (from the platform). At this time, the circumstances are rather favourable," Jacques-Emmanuel Saulnier, Total's head of communication at Total said in an interview published on its website.

"A gas cloud is always a fire hazard," he added.

Total kept two fire-fighting ships in a state of readiness outside a two-mile exclusion zone, which was set up to protect marine traffic, a Total spokeswoman said.

The company has also brought in a robot vessel, not yet deployed, to scan the sea bed for signs of spillage, she said.

The leak started on Sunday and forced the evacuation of all 238 workers from the platform, which sits in waters less than 100 meters deep and 240 km (150 miles) off the east coast of Scotland.

Total as well as UK authorities have described the expected environmental impact from the plume of gas and a spreading sheen of light oil on the water as "minimal", although environmental experts said much of the gas "cocktail" would be either flammable or poisonous at close quarters.

Pressure seen for relief well

The gas leaking from the platform above sea level is coming out of a pipeline Total sealed off from its main Elgin production reservoirs one year ago, the company said.

"What we know is that the leak is not coming from a well dug by Total but from a naturally occurring pocket of gas located just above one of our wells," Saulnier said.

Total said this formation of rock was about 4,000 meters below the seabed, over a kilometre above its own production reservoir.

The company did not say what caused the gas from this pocket to enter its pipes and how it reached the surface.

Hauge said that because the leak seemed to come from a pocket with less pressure and less volume of gas than Total's main reservoir, "we move from a worst case scenario to a bad case scenario. But this is not a good case scenario as long as gas is leaking."

Jefferies securities and investment bank said, "We expect the field to be shut down for the rest of 2012 even if the leak is swiftly resolved, due to likely stringent inspections of the other wells on the field."

Total warned on Tuesday it could take six months to halt the flow of gas. The company previously stated it hoped the leak would die down from natural causes as reservoir pressure drops.

The depth of the non-producing reservoir that is feeding gas to the Elgin platform via compromised layers of piping suggests, however, there is more gas present rather than less, piling pressure on Total to drill a relief well, an engineer with knowledge of the matter said.

Relief drilling would require boring through 4 kilometers of rock with painstaking mathematical precision, because it must intercept the gas pocket at exactly the right point, requiring constant alterations in course, the engineer said.

The leak, one of the biggest in the North Sea for decades, could well inspire tougher safety regulation in due course, according to experts. Britain's health and safety watchdog said it was considering launching an investigation into the incident, while union officials said the frequency of offshore safety lapses had become intolerable.

Memories are still raw in the North Sea industry of the Piper Alpha platform fire 24 years ago, which killed 167 people in the world's deadliest offshore oil disaster and led to a major regulatory overhaul.

Analysts said the French oil major could face costs of up to $10 billion if its North Sea gas leak leads to an explosion and nearly $3 billion if it takes months to fix.

However, Jefferies bank said the data that had emerged on the spill had "further convinced us that the spill consequences should be less than the most pessimistic market estimates and hence that the US$9.7 billion sell-off in the stock since Monday is overdone."