The United States demanded the safe release of two American pilots whose Army scout helicopter strayed into North Korean airspace Saturday and was either shot down or forced to make an emergency landing.

Defense Secretary William J. Perry said in a statement that the Pentagon did not know the condition of the two crew members, although North Korean news agencies reported that the pilots had been taken into custody for questioning.Leon E. Panetta, the White House chief of staff, expressed "serious concerns" about the welfare of the crew of what the Pentagon said was an unarmed, two-seat OH-58 Kiowa helicopter.

"We want them returned," Panetta said on the CNN program "Evans & Novak." He added: "It's ominous that this incident took place. This should not have happened."

North Korea said on Saturday that an "enemy helicopter" had been shot down. But administration officials said Saturday it was not clear whether it was shot down or whether the helicopter had been forced down by an operational problem.

And a congressman visiting North Korea was told Saturday morning that the incident was being regarded merely as "unfortunate," Pentagon officials said.

Senior Pentagon officials said the helicopter was on a routine training mission but could not explain why the Kiowa flew three to five miles into North Korean territory.

American air crews are trained to be exceptionally careful flying near the 2.5-mile-wide demilitarized zone that divides the Korean peninsula. Each side of the zone bristles with tanks, artillery and other heavy weapons. North Korea has accused the United States of disguising surveillance operations as training missions.

The Pentagon identified the crewmen as Chief Warrant Officer David Hilemon of Clarksville, Tenn., and Chief Warrant Officer Bobby Hall of Brooksville, Fla.

While Panetta sought to make clear that the administration was taking the incident seriously, it was not yet causing great alarm within the administration.