Under a cloak of secrecy, engineers were believed to be readying a powerful Air Force Titan 4 rocket for blastoff early Sunday on a clandestine mission to put a military spy satellite into orbit, sources said.

The Titan 4, the most powerful unmanned rocket in the nation's inventory, apparently was scheduled for liftoff from the Cape Canaveral Air Force Station shortly before 1 a.m. EDT Sunday, according to government radio traffic monitored by reporters near the launch complex.In addition, flashing red warning lights at the Titan launch complex could be easily seen from nearby public highways and the Coast Guard made frequent broadcasts warning fishermen to steer clear of the rocket's presumed flight path.

But as usual with such military missions, all details about the flight were classified and Air Force officials would neither confirm nor deny reports about an impending launch.

The rocket's payload was classified as well, but it may have been a $180 million Defense Support Program early warning spacecraft built by TRW of Redondo Beach, Calif., capable of detecting rocket launches using a large infrared telescope.

Such a satellite could prove useful in the Persian Gulf crisis, giving the military more warning of any Iraqi missile attacks on U.S. troops or those of its allies.

At the nearby Kennedy Space Center, just a few miles from the Titan's launch pad at the Cape Canaveral Air Force Station, engineers worked through the weekend readying the shuttle Discovery for blastoff next month to carry a European-built solar probe into space.

The shuttle's original launch date was Oct. 5.