The French oil company that operates the leaking North Sea gas platform is considering using a helicopter water drop to extinguish a flare that could ignite the explosive gas cloud.

The news came as reports emerged that platform-owner Total was warned about the risk of a gas leak four weeks ago. The Aberdeen Press and Journal said a local union claims workers' fears about high pressure in the 13,000ft deep reservoir were "fobbed off" a month ago.

The Department of Energy and Climate Change said: "Total has confirmed in their latest update to the government's inspectors that the flare remains lit but observations from the latest aerial surveillance suggest that the flame appears to be reducing in size."

Total, which originally failed to make clear that the flare was still alight, said other options to put it out included firefighting vessels and pumping nitrogen to starve the flame of oxygen.

On Thursday night Total said it had traced the leak to a gas pocket in a rock formation 4km (2.5 miles) below the seabed but 1km above the gas reservoir being tapped by the Elgin platform, which was evacuated on Sunday when the leak was discovered.

In an attempt to quell fears about the risks of explosion from the gas cloud which has settled above the area, it said there were now firefighting ships on standby at the edge of the two nautical mile (2.3 mile) exclusion zone.

Marine experts and oil industry unions are alarmed about the dangers of the gas cloud being ignited by the flare, about 100m from the leak, if the wind changes direction in coming days.

Union leaders have urged oil companies to evacuate every rig and platform within five miles of the Elgin as a precaution. Total said the wind was expected to continue blowing the gas away from the flare, which was well above the gas cloud, and predicted the flare could burn itself out within a few days.

In a statement on Thursday evening, the company said the flare was burning off evaporating liquids still in the platform's system. "As these liquids evaporate the flow of hydrocarbons to the flare will exhaust itself and the flare should burn out," it said.

Total disputed allegations that the gas was leaking from the seabed and that it was toxic. It said the gas was escaping from the deck level of the platform, about 25m above sea level, and was natural gas.

"This means that we now know the source of the leak, we know the problem," a company spokesman said. "We can now look at solving that problem as quickly as possible. All our focus is now on fixing it as soon as we can."