In a third straight day of airstrikes on Iraq, American planes on Thursday fired upon military communications sites, U.S. officials said.

U.S. planes hit three communications sites 15 miles west and 18 miles south of the northern city of Mosul, according to a statement from the U.S. task force that patrols the northern no-fly zone.

The strikes on fiber-optic, microwave and cable communications facilities -- in response to anti-aircraft artillery attacks on no-fly zone patrols -- came at about 2:20 p.m. EST, said the statement from Combined Task Force Operation Northern Watch.

The communications sites help link Iraq's air defenses, the statement said. All the planes involved returned safely to their base at Incirlik, Turkey, the statement said.

On Wednesday, American planes bombed two military communications sites in southern Iraq. A day earlier, U.S. planes struck mobile surface-to-surface missile equipment and a mobile surface-to-air missile launcher in southern Iraq near Basra, about 35 miles from the border with Kuwait. U.S. planes also hit three mobile surface-to-surface missile launchers in northern Iraq on Tuesday.

The airstrikes come as U.S. and British troops are massing in Kuwait in preparation for a possible war to topple Iraqi President Saddam Hussein. Military officials say the surface-to-surface missiles hit this week threaten those troops and ones the United States wants to send to Turkey, Iraq's northern neighbor.

All of the strikes were within the no-fly zones set up after the 1991 Persian Gulf War to keep Saddam's military from attacking opposition Kurds in the north and Shiite Muslims in the south.

Saddam does not recognize the no-fly zones and his forces frequently shoot at the planes patrolling them. Iraq has never shot down a piloted plane in either zone.