A squadron commander from Utah's Hill Air Force Base said Saturday that the destruction of mobile Scud missile launchers was a top priority but that the Iraqis were skilled at hiding them from his planes.

U.S. and British air forces have redoubled efforts to smash fixed and mobile launchers since 11 Scuds slammed into Israel Friday and Saturday."It is our No. 1 priority . . . how long it will take I have no idea," said Air Force Lt. Col. Tom Rackley. "People must understand it is extremely difficult to locate these missile sites.

"They (the Iraqis) put them in hardened shelters and then under cover of darkness pull them out and move them into some area and shoot them," said Rackley, commander of the 421st Tactical Fighter Squadron.

He said his squadron, based at Hill Air Force Base in Utah, carried out a mission against mobile Scud bases Friday night.

"It is going to be difficult to get them all. It's like trying to get a pea under a pod."

A senior U.S. military officer in Riyadh estimated Saturday that about 30 permanent and 20 mobile missile sites were still operational.

Rackley spoke to reporters at an air base in the southern Arabian Peninsula as dozens of U.S. F-16 fighter-bombers thundered into the sky on a new mission.

Each carried two 2,000-pound bombs and Sidewinder air-to-air missiles. Under rules issued to reporters, the base's location could not be revealed.

Rackley described his squadron's missions so far as "flawless" but said his pilots had been warned against complacency.

Technicians and some pilots agreed that, three days after going into action early Thursday, the missions had become routine. The pilots, only two of whom had been involved in combat before, were gaining confidence, Rackley said.

But he added: "It might be routine for the technicians because they load us up with bombs and we go and come back again. But, certainly, it is not become routine for us (pilots), nor will I allow it to become routine."

"When things become routine, you have a tendency to let down your guard."

Lt. Col. Mark Welsh, commander of the 4th Tactical Fighter Squadron, said his squadron had encountered Iraqi anti-aircraft artillery and surface-to-air missiles, but "we have not run into anything substantial."