Hopes dimmed that a U.S. sailor who fell overboard from an aircraft carrier in the Arabian sea would be found alive after hours of searching produced no signs of survival.

The sailor fell from the USS Kitty Hawk at 7:22 a.m. EST on Wednesday and a search and rescue operation was begun immediately, said Navy Lt. Cmdr. Don Sewell, a Pentagon spokesman.

"We don't know how long the search is going to continue at this point," Sewell said. "But we are still proceeding as though the sailor is alive."

The search was still underway nearly 24 hours later, with no indication it would be called off soon. The reluctance to give up the search for a sailor is not unusual, especially in warm waters like those of the Arabian Sea.

Men and women assigned to aircraft carriers are taught how to survive falling overboard and are considered able to live for days in rough waters, without aid. In 1995, a 20-year-old Marine who fell overboard used his pants to make a float and drifted for 36 hours before being rescued by a Pakistani fisherman.

It was not known how the sailor fell overboard Wednesday.

The sailor was not identified immediately because the family had not yet been notified.

The Kitty Hawk is supporting the U.S. effort in Afghanistan. The ship is part of a three ship battle group being used as a helicopter base for special operations troops.

Before Wednesday, three American servicemen had been reported killed since the U.S.-led airstrikes against the Taliban and terrorist groups in Afghanistan began on Oct. 7.

Two Army Rangers were killed when their Black Hawk crashed in Pakistan on Oct. 19 during support for a special forces raid in Afghanistan — the first combat-related deaths in the campaign. Military officials have not released details of the Black Hawk's mission, but some believed it was preparing to cross into Afghanistan in the event any Army Rangers had to be rescued.

Master Sgt. Evander Earl Andrews was killed Oct. 10 in a heavy equipment accident Qatar.