Platform operator Statoil said 36 well and drilling experts stayed behind on the platform to stabilise the well by pumping heavy drilling mud into the injection well that was still leaking gas.

"The people left are working to calm the pressure and stabilise the well in order to stop the leak," said spokesman Ola Morten Aanestad at Statoil, Norway's biggest energy group.

"It is still a serious situation," he added.

There were no reports of injuries in the rescue operation.

Statoil said the leak forced closure of the 130,000 barrels per day (bpd) Snorre A oil platform and shutdown of the 75,000 bpd Vigdis field nearby from about 1900 GMT on Sunday.

It was unclear when production would resume. Output at the Snorre B platform, which is several kilometres away, continued.

Statoil, partly owned by the Norwegian state, said workers were pumping heavy drilling mud into the injection well which is normally used to inject water and gas into the well.

"We have not succeeded yet," Statoil spokesman Kristofer Hetland said. "The well is still leaking gas.

Statoil said it was planning to send new crew to the platform to replace those who had been working overnight.

WINDS

Gas was leaking from the well to the sea surface but winds blew away the gas before it reached the topside, Statoil said.

Most gas at Snorre A, a steel-hulled tension leg platform, is reinjected into the well. The platform is an integrated production, drilling and quarters (PDQ) unit, anchored to the seabed by tethers.

Statoil said the leak arose during preparations to drill a sidetrack from injection well P-31 on the tension leg platform.

"Although heavy mud had been introduced in advance to stabilise downhole conditions, gas nevertheless entered the mud when production tubing was pulled out," Statoil said.

"That intrusion was registered topside, together with a gas leak from the seabed equipment," it said in a statement.

Snorre has been in production since August 1992.

"This is a very unusual and serious incident," said Oivind Reinertsen, Statoil senior vice president for Statoil's North Sea Tampen area, where Snorre is located.

Oil and gas output from Snorre A are piped to Statoil's Statfjord A platform with crude loaded onto shuttle tankers and gas transported to continental Europe via the Statpipe/Norpipe trunkline.

Norway is the world's number three oil exporter after Saudi Arabia and Russia, producing about three million bpd.

Norwegian state oil group Petoro has the biggest stake in Snorre with 30 percent, followed by Norsk Hydro (17.65 percent), Statoil (14.39), Exxon Mobil (11.15), Idemitsu (9.60), RWE Dea (8.87), Total (5.95), Amerada Hess (1.18) and Enterprise Oil (1.18).