KUWAIT, JULY 22 -- Two American-flagged vessels accompanied by three U.S. warships passed through the Strait of Hormuz and about 12 miles off the Iranian coastline without incident today in the first stage of the controversial U.S. plan to protect Kuwaiti oil tankers from hostilities in the Persian Gulf.

The three American warships, on high alert and bristling with weapons and sensors, lined up in formation with the two Kuwaiti tankers and steamed against a stiff wind and choppy seas into the strait and within range of Iranian Silkworm missile sites. The narrow Strait of Hormuz was considered the most dangerous part of the convoy's voyage to Kuwait's oil terminal.

{In Washington, the Pentagon confirmed the successful transit into the Persian Gulf, United Press International reported. "We're relieved that they're out of the Silkworm missile envelope," Pentagon spokesman Robert Sims said.

{Iranian planes were in the air, but showed no hostile intent as the convoy proceeded through the strait, Sims told reporters.}

The Reagan administration has warned it will attack if the Iranian missiles are deployed and their radars activated while U.S.-flagged vessels are within range.

Cmdr. Daniel J. Murphy Jr., skipper of the destroyer USS Kidd, told his 380-member crew, "It's pretty quiet right now," in a midafternoon assessment summing up the passage through the strait into the gulf.

By late tonight the convoy was deep inside the gulf and it had been joined by a fourth Navy warship, the cruiser Reeves. That ship was added by Navy officials to help protect the two tankers during the night.

While Iran refrained from military activity for the passage of the first American convoy, Iranian President Ali Khamenei said his country would "strike a blow to the ominous alliance" between the United States and Kuwait. He was quoted in a dispatch by the Islamic Republic News Agency monitored in Cyprus.

At the same time, Iran's ambassador to the United Nations, Said Rajaie Khorassani, said in an interview on ABC's "Good Morning America" that Kuwaiti vessels would only become targets if Iraq resumes its attacks on Iranian oil transports and facilities in the gulf.

There have been no shipping attacks in the gulf for more than a week and Iraq has announced it will support a cease-fire resolution passed Monday by the U.N. Security Council.

Navy Capt. David P. Yonkers, who commands the Navy's Destroyer Squadron 14, received a presidential order to begin the operation last night.

At about 9:15 p.m., Cmdr. Murphy of the Kidd, Yonkers' flagship for the convoy, spoke over the ship's public address system:

"This is the captain," he said. "I just wanted to pass to you that we have received word via radio that, in fact, the president has signed the executive order and that, in fact, we will commence the escort operation tomorrow morning."

The convoy weighed anchor at about 9:30 a.m. today (1:30 a.m. EDT) from its assembly area 13 miles off the eastern coast of the United Arab Emirates' port of Khor Fakkan in the Gulf of Oman.

The naval vessels went to an Alpha 1 state of alert, a condition in which about 60 percent of the crew are at battle stations and one alert stage below "general quarters." The Kidd later went to general quarters when it passed the Iranian missile sites, according to a report by a member of the Pentagon media pool aboard the ship.

As the convoy approached the strait, the USS Fox, a guided missile cruiser, took the point position.

"Remember, this is the real thing -- this is not a drill," Capt. William Mathis said over the ship's loudspeakers.

The Fox was followed by the "reflagged" Bridgeton, one of the world's largest supertankers.

The Bridgeton and the second tanker, the Gas Prince, were separated by about one nautical mile. The Bridgeton was in the lead because it is far larger than the Gas Prince and thus is less maneuverable, Yonkers explained.

Aboard the Fox, Petty Officer Allan Lefebure of Cedar Rapids, Iowa, told a member of the Pentagon pool he was in a state of "high anxiety" as the ships approached the strait.

"I think everybody here has the feeling that something is going to happen," he said. "I'm just trying to talk about it with other sailors and keeping my mind busy so I don't think about it too much."

The convoy maintained radio silence and steamed at 16 knots, gulf shipping sources said. There was no indication that any of the ships were hailed by Iranian naval vessels, which have stepped up their challenges to ships entering the gulf.

The two deep water channels of the strait, both in Oman's territorial waters and patrolled by its Navy, are about two miles wide and 330 feet deep. Both the Bridgeton and the Gas Prince were riding high in the water because they are empty. The dull red band of bottom paint that is under water when they are fully loaded was visible on both ships today.

U.S. Navy jets from the aircraft carrier USS Constellation flew continuous patrols over the convoy. And, sources here said, both surface ships and airborne electronic warfare planes scoured the Iranian coast with active and passive radar systems searching for any indications that Iranians were turning on weapons systems that could threaten the convoy.