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ABOARD THE USS EISENHOWER (AP) _ U.S. Navy ships closely watched two Iraqi tankers late Saturday after the oil-carrying vessels ignored U.S. warning shots and headed out of the Persian Gulf area, U.S. military officials said.

A Navy officer told reporters with Defense Secretary Dick Cheney aboard the USS Eisenhower in the Red Sea that the guided-missile frigate USS Reid fired the first shots of the two-day-old interdiction effort ordered by President Bush.

The Pentagon later confirmed two Iraqi ships were warned with gunfire in separate incidents and that both continued under the watch of U.S. vessels. The Pentagon statement said both tankers were believed carrying Iraqi oil.

Officials aboard the ships said U.S. forces briefly went to battle stations over the incidents. The U.S. operation is intended to halt goods leaving and entering Iraq in the Persian Gulf crisis.

''Both tankers are currently under way, but are under close U.S. Navy surveillance,'' according to a Pentagon statement issued late Saturday, when it was already past midnight in the gulf.

A Pentagon official in Washington, speaking on condition of anonymity, said the Navy warships were unlikely to use any force against the tankers during nighttime hours.

The USS Reid fired six warning shots across the bow of an Iraqi tanker that it intercepted in the Gulf of Oman, the Pentagon said.

''Initial reports indicate the Iraqi-flagged vessel Khanaqin refused repeated requests to halt,'' and the Reid then fired the warning shots, the Pentagon said.

Also, the USS Bradley fired three warning shots across the bow of another Iraqi tanker after it refused U.S. requests to halt, the Pentagon said. The unidentified tanker was in the Persian Gulf, heading toward the Gulf of Oman.

The only Iraqi port from which oil could be shipped is at the northern tip of the Persian Gulf, connected to the Gulf of Oman through the Strait of Hormuz. The gulf opens up into the Arabian Sea where oil-laden ships fan out to world ports. The Eisenhower is on the other side of Saudi Arabia in the Red Sea.

In Baghdad, Iraq reported that the United States had fired on two Iraqi oil tankers that refused to be searched, and said a similar act in the future would have ''grave consequences.''

In the Red Sea, Capt. Morris Foote of the guided missile cruiser Ticonderoga said, ''One of our frigates, out in the Gulf of Oman, has fired a warning shot at an Iraqi tanker, in fact, fired six of them.''

The Ticonderoga was operating with the carrier Eisenhower in the Red Sea on the opposite side of the Saudi Arabian peninsula from the shooting incident.

Foote later clarified that statement to say the shots were fired across the bow of the tanker.

Foote said the Persian Gulf forces initially went to battle stations. The Eisenhower battle group, however, did not go on high alert status during the shooting incident, and he said the Iraqi tanker posed no apparent danger to U.S. forces.

The Reid incident occurred about 4 p.m. local time (9 a.m. EDT), south of the narrow entrance to the Persian Gulf from the Gulf of Oman.

Foote did not have details but other officers said they believed the Iraqi ship had not stopped when ordered. Foote said warning shots were fired by the Reid's 76 millimeter gun after the frigate's commander received permission to do so from Rear Adm. William Fogarty, commander of the Middle East task force in the Persian Gulf.

Iraqi Oil Minister Issam Chalabi said in Baghdad that U.S. warships had fired on the Iraqi oil tanker Baba Karkar in the Persian Gulf and the oil tanker Khanaqin in the Gulf of Oman.

Chalabi said the shots fell in front and behind the ships and they proceeded without stopping.

Foote said there was apparently no threat from the Iraqi tanker in the Gulf of Oman, which like all other commercial vessels should not be armed.

''I don't think that there was any other provocation. I don't think there was anything, in fact, threatening from the Iraqis or from any other source,'' he said.

''But as a general course, when we start getting into the business of firing shots, you're never sure of the reaction,'' he said.

On Thursday evening, President Bush authorized American warships to use force if necessary to stop commerce headed to and from Iraq and Kuwait, which is occupied by an estimated 160,000 Iraqi troops.

Defense Department spokesman Pete Williams, accompanying Cheney aboard the Eisenhower at the beginning of a four-day trip to the region, refused to comment on the situation. Cheney was not available for comment.

While on board the destroyer USS Scott, Cheney was told that the Scott had just returned from its interdiction duty at the mouth of the Gulf of Aqaba and had turned back a Cypriot bulk freighter headed for the port.

The captain of the Scott said the Cypriot freighter was headed for the port of Aqaba and had departed from Port Sudan, Sudan's main Red Sea port.

Scott Commander Tom Corcoran said he had a ''very polite conversation'' with the master of the Cypriot ship and the freighter turned around after reporting that it was carrying a cargo of aluminum chromate and other chemicals.

''We didn't want to take the chance. We maneuvered at a safe but fairly close distance and spoke on the radio,'' Corcoran told pool reporters.

He said the ship turned around and headed for an unknown destination.

Cheney was to remain on the Eisenhower until Sunday morning when he was to travel to northern Saudi Arabia and visit American troops deployed on land in the defense of the desert kingdoms' vital oil fields.

He was to travel on to Bahrain, and several other Persian Gulf nations before visiting in Jiddah on Monday and stopping in Cairo before arriving back in Washington early Wednesday.