A pipeline that leaked oily material onto the tundra at BP‘s oil field in Alaska was filled with extensive ice plugs, state officials have announced.

The ice build up within the line was detected by X-ray analysis launched by BP, the Alaska Department of Environmental Conservation said in a statement, reports Reuters.

It is not clear whether the ice plugs, one of which was around 1,500ft long, caused the leak, the spokeswoman for the department is reported to have said.

The spill and the cleanup work have not affected oil production at Lisburne, whose output is about 35,000 barrels bpd, BP spokesman Steve Rinehart told Reuters.

The leak in the pipeline, an 18in diameter flow line, was discovered last week. Although it had been taken off service weeks earlier because of suspected ice plugs, it was still pressurised and carrying material when the leak was found.

A mixture of crude oil, oil- and salt-laced produced water and natural gas flowed out of the leak and sprayed an estimated 8,400ft2 of snow-covered tundra, according to state environmental officials and representatives from BP Exploration (Alaska).

The leak presents potential legal problems for BP, which remains under a term of a three-year probation as part of a 2007 federal criminal settlement concerning a major pipeline spill at Prudhoe Bay in 2006.