KUWAIT, JULY 21 -- American flags were unfurled today on the sterns of two Kuwaiti-owned tankers as they prepared to weigh anchor and begin their inaugural passage under heavy U.S. Navy escort through the Strait of Hormuz and into the Persian Gulf.

{Japanese shipping sources said early Wednesday that the first tanker got under way at 8:07 local time and was followed by the second about one hour later, Reuter reported.}

"This is a milestone," said U.S Ambassador Anthony Quainton. "We are providing direct reassurance to one of the small countries in the gulf that has found itself in danger from the Iran-Iraq war."

Meanwhile, the French and British navies dispatched more warships to the gulf. France, which has a large Indian Ocean fleet based at Djibouti, said today it will give military escort to two oil tankers sailing into the Persian Gulf this week.

Sources here said France will step up its monthly patrols of the gulf to twice a month.

The U.S. Congress, some of whose members have expressed concern that U.S. intervention will lead to a direct confrontation with Iran, sought unsuccessfully to delay the start of the escort operation.

One of the larger mobilizations in recent years of combat-ready U.S. forces has been assembled for the convoy's voyage to Kuwait's Ahmadi oil terminal. Four U.S. warships will form a convoy with the reflagged Kuwaiti vessels. Overhead, U.S. and Saudi Airborne Warning and Control System surveillance planes will keep watch, along with naval reconnaissance P3 Orions flying out of Oman.

Fifty carrier-based warplanes and electronic-jamming aircraft aboard the USS Constellation in the Arabian Sea are poised to counter a variety of potential threats against the Kuwaiti tankers and their escorts, including land-based Silkworm missiles Iran has purchased from China and Iranian naval attack boats.

Greater secrecy has surrounded the movement of a U.S. helicopter carrier, the USS Guadalcanal, and its contingent of 1,800 Marines as well as the battleship USS Missouri, both of which have passed through the Suez Canal in recent weeks to assume stations near the Arabian Peninsula.

Members of the Pentagon-sponsored media pool aboard the cruiser USS Fox said Navy officials escorted a launch with five reporters from the Fox to one of the tankers, the Gas Prince, to witness the flag-raising. Navy officials were eager to position the Gas Prince between the reporters' launch and the Fox so the cruiser's hull would offer a dramatic photographic backdrop to the ceremony. Encouraged by photographers, the Fox maneuvered several times around the Gas Prince to ensure the best pictures.

Iran today criticized a U.N. Security Council resolution passed yesterday calling for a cease-fire in the seven-year-old gulf war. The resolution holds out the possibility of sanctions against either country if it fails to comply.

"The resolution is unjust," said an Iranian Foreign Ministry statement. "The intervention of U.S. forces in the gulf is a blatant violation of the resolution and renders it null and void."

{At the United Nations, Iranian Ambassador Said Rajaie Khorasssani called the reflagging of the tankers the first violation of the resolution and "the most serious escalation of the conflict," United Press International reported.}

Nevertheless, western analysts based here and high-level Kuwaiti officials were known to believe that Iran will cease its attacks for an indefinite period and continue taking a very cautious approach to the U.S. naval presence in the gulf.

The head of the U.S. naval convoy, Capt. David P. Yonkers, told pool reporters aboard the destroyer USS Kidd that he did not believe Iranian missiles would be used against the tankers. "If they were to launch one, that would probably be the last one," Yonkers said.

"I think the Iranians will be very careful," said one western diplomat. There have been no Iranian attacks on shipping for the past eight days although Tehran radio announced today that a naval patrol by Iran's Revolutionary Guards seized the crews of three vessels it called "Kuwaiti spy boats" in the gulf.

The report did not say when or where the ships' crews were seized, and Kuwaiti officials did not confirm the report.

{Arab diplomatic sources reported that the United States, while it has not formally asked Iraq to stop its tanker attacks, is discussing "some gestures" the Iraqis could make to facilitate a possible end to the fighting. Under discussion, the sources said, are various measures Iraq could take at least on a temporary basis, such as a suspension of attacks on Iran-bound oil tankers, to decrease gulf tensions.

{In Washington, Iraq's ambassador to the United States, Nizar Hamdoon, said that his government would only accept the U.N. demand for a cease-fire if it was linked to a withdrawal of Iranian forces to internationally recognized borders between the two warring nations.

{He said Iraq "cannot accept" just a standstill cease-fire that would leave Iran forces still in control of some Iraqi lands.

{Hamdoon said the Iraqi understanding of the U.N. resolution is that it stands as a package in which a cease-fire is called to allow for the withdrawal of the two nation's troops to those borders and for negotiations to end the war.

{Hamdoon also said Iraq would not agree to end unilaterally its attacks on Iran-bound oil tankers unless Iran agreed to the other terms of the U.N. resolution. In the past, Iraq has said it will not agree to end the "tanker war" until Iran agrees to allow Iraq to use its blockaded Persian Gulf ports again.}

The other reflagged Kuwaiti ship in the first convoy is the Bridgeton, one of the largest ships in the world. Its new American captain was identified as Frank Seitz, and the captain of the Gas Prince was identified as Joseph Roach.

Nearby, in the Gulf of Oman, the USS Kidd and USS Fox also stood ready to get under way as temperatures soared over 100 degrees on the sun-broiled gulf.

U.S. officials apparently have made it clear that if Iran activates Silkworm missiles at launching sites overlooking the Strait of Hormuz, the act of turning on targeting radars will be taken by the United States as a show of "hostile intent" and will subject the missile sites to attack.

It is not known how U.S. escorts will react to challenges by Iranian naval vessels seeking to stop war materiel headed to Iraq. U.S. Navy officials in the gulf have said in the past that Iran has rights under international law to stop, and perhaps inspect, commercial vessels entering the gulf.

In recent weeks, Iran has stepped up its challenges to ships at the mouth of the gulf.

There were strong indications that the United States was deploying a number of forces to the Middle East that could be injected quickly into the area to meet "worst case" scenarios of major confrontation with Iran sparked by the tension-filled entry of the U.S. and other naval forces into the gulf.

A U.S. Air Force squadron of F16 interceptors was said to be en route to Jordan. Officials said, however, that the planes were to take part in exercises planned well before the dates were set for the American escort operation.

Within the gulf, the island state of Bahrain, located between Kuwait and the mouth of the gulf, already provides major naval facilities for the U.S. Middle East Force fleet. Aircraft flying from U.S. carriers outside the gulf cannot easily extend umbrella protection to the convoys as they approach Kuwait.

Recently, the emirate contracted for a $90 million air base for its small Air Force, which ultimately would include 12 F16s it hopes to purchase with U.S. military aid loans.Washington Post staff writers David B. Ottaway and R. Jeffrey Smith contributed to this report.