ABOARD THE KHARG, AUG. 19 -- The American-made helicopters dragged mine-hunting sonar devices through the choppy waters of the Persian Gulf as their rotors slashed the muggy air.

Pilots and crewmen, trained in Florida, Texas and Virginia, conversed in English over U.S.-built radios, occasionally flashing the thumbs-up sign to make their approval clear over the din of the motors.

To all appearances, the mine-sweeping action seemed part of the American naval mission to make the gulf's treacherous waters safe for the transport of Kuwaiti oil. But in reality, the operation was being conducted by Iranian sailors employing U.S. equipment purchased during the era of the shah and tactics learned when Iran and the United States were close military allies.

To the two dozen foreign reporters flown here by the Iranian Navy to observe Iran's mine-sweeping task force at work in the Strait of Hormuz, the banter, attitudes and equipment displayed by the Iranian forces appeared strikingly similar to those of U.S. crewmen toiling nearby -- on ships that fly the American flag instead of the banner of Iran's Islamic revolution.

As Iranian RH53 Sea Stallion helicopters and smaller rotor craft dragged sonar mine-hunting equipment through the gulf waters, others lowered devices into the sea to detonate acoustical mines or drop depth charges in a tactic the Iranian force commander, Capt. Ali Izadi, said would explode contact mines.

Iranian sailors identified the USS Flatley, a guided-missile frigate, off in the distant haze, the only gray profile breaking the monotony of the horizon in the Gulf of Oman.

The Flatley, steaming along close enough to observe this Iranian mine-sweeping force, is a highly visible sign of U.S. naval presence here at the entrance to the Persian Gulf, a presence Iran's Islamic revolutionary leadership says is unnecessary and dangerous.

For the last several days Iran has been dramatizing its point with a unilateral mine-sweeping effort in international waters about 15 miles east of the gulf emirate of Fujayrah. Capt. Faramarz Khoshmanesh, a Navy liaison officer, said the force of six ships and six helicopters has discovered four mines so far.

"Our mission is to sweep the area of mines," said Cmdr. Rahmat Taheri, operations officer aboard this converted Iranian supply vessel that serves as the task force's command ship. "We have no idea who planted the mines."

Western governments have expressed belief that Ayatollah Ruhollah Khomeini's government in Tehran laid the mines as part of its seven-year-old war with Iraq. Encouraging this belief, Khomeini's followers have warned in recent days that if Iraq resumes attacks on Iranian oil shipments through the gulf, Iranian forces will make sure no nation can use the waterways safely.

For the time being, however, the Iranian government has emphasized its desire to see the gulf remain open for navigation. Tehran needs the shipping lanes desperately to continue enough oil exports to finance the costly military struggle against Iraq.

Iran today strongly denied that its patrol boats were involved in an attack yesterday on the Liberian-registered chemical tanker Osco Sierra. "The Islamic Republic of Iran has announced time and again that as long as the Iraqi regime refrains from attacking ships, Iran will not embark on retaliatory measures," the official Iranian news agency quoted a government spokesman as saying.

The Iranian Navy formally offered last week to work with the United Arab Emirates to keep these waters safe without involving the U.S. and European mine-sweeping equipment sent to the area after mines were found. The U.A.E. declined the offer.

The Iranian Navy then dispatched its task force unilaterally, declaring it has the men, experience and equipment to make the outside powers unnecessary in the gulf.

Massoud Khadarati, who piloted the reporters' Sea Stallion, hovered over the water while a neighboring helicopter dropped its sonar equipment into the sea to send out beams in search of mines below the waterline.

But the devices turned up no mines.

Similarly, another craft sent out acoustical beams in the surrounding water, but without detonating any mines. Taheri, in a briefing for reporters, said the few mines discovered so far were contact mines moored below the surface designed to detonate when struck by passing vessels.

Taheri said Iran had been using coastal ships and expert divers to search for mines until today, when helicopters joined in the search. He said Iran was sweeping for mines in waters 10 miles east of the U.A.E. coastline, from Fujayrah to the Khor Fakkan anchorage.

The Iranian government repeatedly has boasted that its men and equipment are as good as any. Sailors and pilots recalled today that they cleared Iranian ports at the northern end of the gulf of mines laid by Iraq four years ago at the opening of the tanker war.