ABOARD THE USS SAMUEL B. ROBERTS (AP) _ The U.S. Navy is preparing to shepherd its 40th convoy through the Persian Gulf war zone and officers say alertness is as high as when the operation began eight months ago.

They said, however, that a permanent high level of alertness takes a toll on the crew.

''The hardest part of the mission is that you have to stay ready 24 hours a day. There's no time, really, to let your guard down,'' said Cmdr. Paul Rinn, 40, of the Bronx, N.Y.

He commands this missile frigate, one of 16 Navy ships serving in the escort and patrol squadron called the Middle East Force.

Rinn spoke with members of a Pentagon news media pool Tuesday on the bridge of his ship, one of the Navy's newest Perry-class frigates.

Soon afterward, officers in the combat information center were monitoring radio traffic about an Iranian attack on a commercial vessel off Dubai, 90 miles away.

Messages said the Greek-flagged tanker Stavros G.L. was attacked at 2:45 p.m. approximately 30 miles north of Dubai and was on fire.

The U.S. frigate Jack Williams, a sister ship to the Roberts, went to the tanker's aid from about nine miles away. The Williams rescued 14 crewmen in a lifeboat, according to Lt. Cmdr. Mark Van Dyke, a Navy spokesman.

When the fire was under control, the tanker's skipper asked that the crew be returned to the ship, Van Dyke said.

It was believed to be the third time a U.S. warship gave ''humanitarian assistance'' to a ship attacked by Iranian gunboats in the gulf. The last was Christmas Day when the USS Elrod rescued crewmen from a burning vessel in the southern gulf.

Iran has attacked seven neutral ships in the gulf since Friday. Iraq has raided a similar number of Iranian owned or chartered tankers beginning earlier in March. Iraq and Iran have been at war since September 1980.

The Roberts, a 445-foot vessel launched two years ago, began its latest mission after a three-day resupply stop and the first liberty for the 209-man crew since arriving in the gulf nearly a month ago.

Soon after entering international waters Tuesday, Rinn ordered the crew to general quarters to ''clear the mind'' after the port call.

They test-fired defensive gun systems as part of the effort to renew vigilance.

On its last mission in the northern gulf, the $400 million warship had a series of ''close encounters'' with Iraqi aircraft, the captain said.

From 30 to 40 Iraqi air missions came near his ship en route to attacks on Iranian vessels, Rinn said, including three Mirage F-1s armed with Exocet missiles that flew wing-to-wing 50 feet above the water.

He said the jets came within five miles of the Roberts, heading away and not demonstrating hostile intent.

''You can basically say we were a blink of an eye away from anything that could happen,'' Rinn said.

''I'm never going to allow my ship to be shot at and hit,'' said Rinn, adding the fate of another sister ship, the USS Stark, ''is not lost on us.'' The Stark lost 37 of its crew when hit by an Iraqi Exocet missile last May 17.