Red Cross chief Peter Maurer on Tuesday condemned US drone strikes outside areas officially engulfed in armed conflict, warning against a creeping expansion of the definition of what constitutes a battlefield.

Washington's secretive and controversial use of drones was not a problem in itself, said Maurer, as in the context of an armed conflict drones are considered legitimate weapons.

"But if a drone is used in a country where there is no armed conflict... there is a problem," the head of the International Committee of the Red Cross told reporters in Geneva, urging the "very restrained use" of the weapon.

"A drone used in Afghanistan or Yemen is a drone used within the context of an armed conflict, and is thereby used legitimately," he said. But drone use in Pakistan was "particularly problematic" he said.

Returning from a trip to the United States, where he met with President Barack Obama, Maurer told AFP that "the US is very aware... of where we disagree with the use of drones".

The main problem with drone strikes today is a widening interpretation of what constitutes a battlefield, he said.

"To link (the definition of) battlefields to combattants on the move is an interpretation that we don't share," he said.

Lawmakers and rights advocates have criticised the US for its strikes against suspected Al-Qaeda militants in Pakistan, Yemen and elsewhere, but US officials refuse to publicly discuss any details of the covert campaign.

Maurer said he had also used his trip to urge Washington to swiftly address the problem of Guantanamo, where dozens of prisoners have been staging a hunger strike since February.

Obama moved to close the controversial US detention facility in 2009, but plans to try suspects in US civilian courts were stymied by Congress, leaving many inmates in limbo.

"I think the lack of perspective in terms of transfer... is at the origin of the big malaise that has been transformed into a hunger strike," Maurer said.

While the Red Cross regularly visits the Guantanamo detainees, its reports on the prison's conditions remain privy to the US government.