A team of experts spent four hours inspecting a wellhead that is spewing a cloud of potentially explosive gas on a North Sea platform operated by French energy giant Total, the company said on Thursday.

It was the first time the wellhead has been inspected since the Elgin platform was evacuated of its 238 crew on March 25 when the leak was detected.

Total said a helicopter carrying the eight-strong team of experts left Aberdeen on the east coast of Scotland at 10:30 am (0930 GMT) to fly the 150 miles (240 kilometres) to the rig and returned at 4:50 pm.

In a "key milestone" towards bringing the situation under control, the team spent nearly four hours establishing which zones of the platform were safe to access and what equipment will be required in any well control operation.

The platform is surrounded by a cloud of low-lying gas and Andrew Hogg, Total's UK communications manager, said Wednesday there was a continuing risk of explosion.

Total is due to begin a two-pronged operation to block the well by pumping it full of "heavy mud" at high pressure and drilling two relief wells.

Jacques-Emmanuel Saulnier, Total's head of communications, said the survey would ensure that the teams who will later work on blocking the wells "can do so in the best possible conditions".

Strong winds had thwarted plans to fly the experts to the platform on Wednesday.

An estimated 200,000 cubic metres of highly flammable gas are escaping from the platform each day in a leak which Total says is costing the company $2.5 million (1.91 million euros) daily.

The assessment team included staff from the rig who are familiar with its construction, as well as outside experts from Texas-based firm Wild Well Control.

The company 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.

A Greenpeace ship sailed to the vicinity of the Elgin platform on Monday to take air and water samples.

Activists from the environmental group said they saw an oily substance on the surface of the water around the platform, but Total insisted it was caused by gas condensate and not oil.