U.S. Defense Secretary Leon Panetta said on Friday militants who attacked the United States and its citizens will hunted down, in the first comments by a senior U.S. official on a hostage attack by Islamist militants in Algeria.

He said the U.S. government was working around the clock to ensure the safe return of its citizens caught up in the Algeria crisis.

A U.S. plane landed on Friday at an airport near a desert gas plant in Algeria where Islamist gunmen took hundreds of hostages to evacuate Americans caught up in the crisis, a local source said.

"Regardless of the motivation of the hostage takers, there is no justification, no justification for the kidnapping and murder of innocent people," Panetta said during a visit to London, adding that the United States was in close touch with the Algerian government about the crisis.

"Terrorists should be on notice that they will find no sanctuary, no refuge, not in Algeria, not in North Africa, not anywhere. Those who would wantonly attack our country and our people will have no place to hide," he said.

U.S. officials had no clear information on the fate of Americans, though a U.S. military drone had flown over the gas complex where hostage crisis unfolded.