WASHINGTON (Reuters) - Royal Dutch Shell may have moved the Kulluk oil rig that ran aground off Alaska last month to avoid $6 million in taxes, Congressman Ed Markey said on Thursday, disputing the company's claims it moved the rig because of weather forecasts.

The 30-year old rig ran aground on New Year's Eve in what were described as "near hurricane" conditions while it was being towed south for the winter.

Markey, in a letter sent to Shell President Marvin Odum on Wednesday, said the decision to move the rig "may have been driven, in part, by a desire to avoid ... tax liability on the rig."

An aide to Markey, an outspoken critic of the oil and gas industry and the top Democrat on the House Natural Resources committee, said the lawmaker received information about Shell and taxes from the Alaska revenue department.

The U.S. Interior Department said this week it will review Shell's Arctic oil drilling program, on which the company has spent $4.5 billion since 2005, to assess challenges and guide future permitting in the region.

Shell did not immediately respond to a request for comment.

(Reporting by Timothy Gardner; editing by John Wallace and Jim Marshall)