RIYADH, Saudi Arabia (AP) _ Two U.S. warships struck floating mines in the northern Persian Gulf today, and the Navy said seven servicemen were injured. Both vessels were damaged but were not in danger of sinking, officials said.

Initial reports said the USS Tripoli, a helicopter-carrying amphibious assault ship, and the USS Princeton, a 2-year-old, high-tech missile cruiser, were operating under their own power after the explosions.

Lt. Commander Ken Satterfield, a Navy spokesman, said three sailors were injured aboard the Princeton - one seriously - and four were slightly injured on the Tripoli.

All those injured on the Princeton were airlifted to another ship for treatment, he said, but those injured on the Tripoli were treated aboard ship.

The injured men were not identified.

It was the first mine damage to U.S. vessels in Persian Gulf deployment - both the month-long war and the preceding five months of enforcing the embargo on trade with Iraq imposed after its seizure of Kuwait.

U.S. officials have said they believe Iraq is dumping floating mines in the gulf. More than 80 mines have been found and destroyed in the past few months, but none had previously damaged a vessel.

The 18,500-ton Tripoli is one of four helicopter carriers in the 31-vessel task force moving north in the gulf in preparation for a possible Marine landing.

Although it normally carries a complement of up to 2,000 combat-ready Marines along with its crew of about 690, military officials indicated it did not necessarily have any such force on board at this time.

The Aegis-class Princeton was sent to the gulf with a aircraft carrier battle group. The 9,500-ton, billion-dollar warship is equipped with a computer-linked radar and missile system for long-range air defense, and has a crew of about 360.

The U.S. Command said the mine incidents occurred about 2 1/2 hours apart, with the 600-foot-long Tripoli being struck first, at about 4:40 a.m.

Marine Brig. Gen. Richard Neal told reporters the Tripoli initially reported two men had gone overboard, and the Princeton was among ships that took part in a search for the sailors.

The 570-foot-long Princeton hit a mine about 10 miles from the Tripoli, Neal said. Initial reports had said the ships were 60 to 120 miles apart.

Neal said officers aboard the Tripoli later determined no sailors were missing.

He said the Princeton was operating at 50 percent power because of damage to one of her propellers. The forward compartment of the Tripoli was damaged and there was some flooding, but the ship was fully operational, he said.

The mine that holed the Tripoli was a contact mine, the general said, and the one that damaged the Princeton apparently was an ''influence mine,'' designed to blow up under the ship and damage its keel. It was the first such mine encountered in the Persian Gulf War.

U.S. military sources said today that many of the mines currently being found in the gulf, while based on a turn-of-the century Russian model, appeared to have been recently manufactured, probably by the Iraqis, and let loose to drift in the busy oil waterway.

The Tripoli, an Iwo Jima-class vessel commissioned in 1966 and based in San Diego, can carry up to 25 helicopters. The Tripoli and the Princeton, which is based at Long Beach, Calif., are among more than 100 Navy ships deployed as part of Operation Desert Storm.

All U.S. warships in the gulf operate with special mine spotters posted on the bow at all times. But the floating mines are often difficult to see, particularly in rough waters.

Senior U.S. naval officers said at the end of the Iraq-Iran war in 1988 that as many as 200 mines might be left in the waters along Iraq's small gulf coast, and that many of them could break loose and drift south on the prevailing currents.

However, U.S. military sources said that most of the mines recently found in the gulf appeared to be of recent manufacture, and were not crusted with barnacles or other marine growth, indicating that they had not been in the water long.

There are several mine hunters and mine sweepers in the Persian Gulf as part of Operation Desert Storm, but U.S. officals have been close-mouthed about their role so far.

Mine sweepers generally do not go out in search of floating mines, but are used to clear areas where mines are known or suspected to have been laid.