A U.S. frigate suffered an underwater explosion in the Persian Gulf Thursday and four American seamen suffered injuries, none of them life-threatening, the Pentagon said.

Spokesman Dan Howard said flooding in the frigate Samuel B. Roberts was under control and the cruiser Wainwright was on its way to aid the Roberts. He said the explosion took place at approximately 10:10 a.m. EDT."We don't know what caused the explosion," Howard said. He said the explosion took place 67.5 miles due east of a gulf navigation point called "Bahrain Bell."

He said the Samuel B. Roberts recently completed an escort of Kuwaiti tankers, recommissioned to fly the American flag, and was on its way back to its station.

There had been no recent evidence of mines in the vicinity, Howard said. This was the first incident involving an underwater explosion since the tanker Bridgeton hit a mine last July on the first escort of re-flagged Kuwaiti tankers.

Howard said he assumed there were no other U.S. vessels in the area since the Wainwright was en route to the scene.

White House spokesman Marlin Fitzwater said the Persian Gulf incident "emphasizes once again our concern for stopping the Iran-Iraq War."

Fitzwater said the president learned about the incident earlier Thursday from the National Security Council.

Howard said the explosion caused flooding in the engine room and some hull damage. "The flooding is under control and the ship is in the process of pumping out water," the spokesman said.

He said the damaged ship was operating under auxiliary power. There are no details of the injuries, he said, but "at the moment it appears none of them are life threatening."

Howard said details would be provided during the day as the situation develops.

The homeport of the Samuel B. Roberts, with a crew of 215, is Newport, R.I. The Wainwright is based in Charleston, S.C.

The explosion marks the first incident in the gulf since last November when the U.S. frigate Carr fired at a suspected Iranian vessel making a run at the U.S. supply ship Patriot.