Air India flew some of its Boeing Co 787 Dreamliner aircraft even after U.S. regulators grounded the global fleet last month due to undiagnosed battery problems.

Dreamliners operated by India's national carrier were flown to Mumbai for maintenance reasons, Arun Mishra, the head of India's civil aviation regulator, said on Monday. A spokesman for state-run Air India declined to comment.

"When the Dreamliners were grounded, they had come to Delhi from Frankfurt and Paris. Air India asked us for permission to take them to Mumbai because they have their maintenance facility there and also they were paying very high parking charges in Delhi," Mishra, director general of civil aviation, told Reuters. "We gave them permission with strict orders that no passenger will be allowed."

The 50 technologically advanced Dreamliners in global service have been grounded since mid-January as officials in the United States, Japan and France investigate a battery fire and a battery failure on two separate planes last month.

U.S. officials said this week they were making progress in their investigations into the battery issues.

India grounded Air India's six Dreamliner jets on January 17 in line with the U.S. Federal Aviation Administration's advisory to ground the aircraft. The national carrier said last month that Boeing would be liable for compensation.

(Reporting by Anurag Kotoky; Editing by Ian Geoghegan)