CAIRO, AUG. 18 -- U.S. Navy ships fired warning shots today across the bows of two Iraqi oil tankers operating in Middle Eastern waters, prompting a threat from Iraq that a repeat of such incidents would have "grave consequences."

The warning volleys in the Persian Gulf and the Gulf of Oman were the first shots fired in the increasingly tense military face-off between U.S. and Iraqi forces in the crisis sparked by Iraq's Aug. 2 invasion of Kuwait.

Tensions heightened further today when the Iraqi government declared that foreign nationals -- including their newborn children -- trapped in Iraq and Kuwait would suffer from any shortages of food and medicine that result from an international trade embargo against Baghdad.

The announcement followed a statement Friday that Iraq was indefinitely detaining thousands of foreign nationals caught in Iraq and Kuwait. The Iraqi foreign minister, Tariq Aziz, made clear today that the foreigners, who are to be housed at strategic military and civilian sites, were being held as insurance against attack by U.S.-led forces massing in the region.

The U.N. Security Council tonight unanimously adopted a resolution demanding that Iraq immediately release the foreigners it is detaining in Kuwait and Iraq and that Baghdad do nothing to jeopardize their safety or health.

U.N. Secretary General Javier Perez de Cuellar, in Lima, Peru, announced that he is sending two high-ranking U.N. officials to Iraq to open talks on the fate of the foreigners being detained by Baghdad. The action was in response to a call Friday by the Security Council for all "appropriate" steps to win their release.

Earlier, Iraq's ambassador to Paris said that some of the 4,700 Britons had been moved to Iraqi military installations. In London, a government spokesman said about 40 Britons had been moved out of their hotels in Kuwait to "unknown destinations."

In a statement issued at President Bush's summer home in Kennebunkport, Maine, White House spokesman Marlin Fitzwater said, "The president is . . . deeply concerned about today's announcement by the government of Iraq that foreign nationals may not have access to adequate quantities of food." There are an estimated 3,000 Americans in Iraq and Kuwait.

Describing Bush's reaction to the announced transfer of the foreigners, Fitzwater said, "The use of innocent civilians as pawns to promote what Iraq sees to be its self-interest is contrary to international law and, indeed, to all accepted norms of international conduct."

The incidents today in the Persian Gulf and the Gulf of Oman, in which naval personnel manned battle stations for several hours, came on the second day of an interdiction effort ordered by Bush to halt all goods leaving and entering Iraq as part of a worldwide effort to isolate the government of President Saddam Hussein.

Pentagon officials said the U.S. Navy frigate USS Reid intercepted the Iraqi-flagged tanker Khanaqin in the Gulf of Oman. After the vessel refused repeated requests to halt, the Reid fired six warning shots across the bow of the tanker, officials said.

In the Persian Gulf, the USS Bradley intercepted an Iraqi tanker and also made repeated requests for the vessel to halt, the officials said. When the tanker refused, the Bradley fired three warning shots across the bow.

Both tankers continued on the high seas under close U.S. Navy surveillance, according to U.S. officials who did not specify in which direction the tankers were headed.

The official Iraqi News Agency quoted Oil Minister Issam Chalabi as saying that U.S. warships had fired on two Iraqi oil tankers: the Baba Karkar in the Persian Gulf and the Khanaqin in the Gulf of Oman. Chalabi said the shots, which came after the ships refused to allow themselves to be searched, did not strike the ships, and the vessels proceeded without stopping.

"The Oil Ministry of the Iraqi republic therefore warns that any interception of its tankers and their crews will result in grave consequences which the American authorities bear responsibility for," he said.

Branding the U.S.-led blockade of ships to Iraq as an act of war, a statement issued in Baghdad by the Ministry of Labor and Social Affairs said that newborn babies of foreigners trapped in Iraq would be as much victims of any food shortages as Iraqi children. Thousands of adult foreigners would share the same hardships, the statement said.

"As the world's babies, irrespective of their nationalities, are God's beloved and the beloved of the leader Saddam Hussein, and in harmony with our humanitarian principles, we have decided to treat babies of foreign families which the people have decided to host fully equally with the babies of Iraq in determining their needs," the statement said.

"Should there be a shortage in babies' milk, medicines and other requirements because of the boycott, then whatever shortages of food, medicines, clothing or anything else affect the babies of Iraq will also affect them equally," it said.

The statement said that priority in food allotments would be given to soldiers and defense workers, adding, "And as the foreigners in question will not be among those engaged in the armed forces or activities related to them, their share of the food, the quantity and type, will be affected along with that of other Iraqis by our imports of food in one way or another."

In an interview with CBS News today, Aziz said that the foreigners being held in Kuwait and Iraq will "live in compounds alongside with the Iraqis, so they will not be prisoners and they will not be treated in a harsh or a hostile manner."

But he made clear that Baghdad was detaining the foreigners in an attempt to forestall attacks on Iraq by U.S. and other forces being deployed in the region.

Iraqi leaders, he said, "think that if a number of foreigners would live with the Iraqis in some places who are the target of probable American aggression, that might deter the American government from committing the crime that the Iraqi people would like to avoid and we hope that the American people would like also to avoid."

Aziz also told CBS that Baghdad would not use chemical weapons against the United States unless Washington used nuclear weapons against Iraq. But in another statement, Iraq's army newspaper threatened "mass destruction" against any country that attacked.

International concern about the foreigners trapped in Kuwait and Iraq has heightened in part because of reports of ill treatment from a flood of people who have escaped into Jordan and Saudi Arabia in often harrowing journeys across the searing desert.

Jordanian Interior Minister Salem Masadeh said today that 100,000 people have fled to his country from Iraq and occupied Kuwait since the Aug. 2 invasion. Today, by mid-morning, another 7,631 people, including five Americans, had crossed into Jordan at the Iraqi frontier post at Ruweished alone.

International efforts to defuse the crisis continued, as defense ministers of the six-state Gulf Cooperation Council announced that they will meet next week to forge a joint strategy on Iraq's occupation of Kuwait. The ministers of Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates plan to meet in Riyadh. No further details were given.

Egypt called today for a special meeting of Arab foreign ministers in Cairo on Aug. 26 to discuss implementation of resolutions that were approved at an Aug. 10 summit led by Egyptian President Hosni Mubarak. It was not clear how many Arab countries would send foreign ministers to the meeting. At the summit, a majority of Arab nations condemned the Iraqi invasion and formed an all-Arab force to face the threat to Saudi Arabia and other gulf countries.

In Maine today, Fitzwater said the United States intends to "consult with other governments with citizens being held in Iraq and Kuwait to determine what additional measures ought to be taken" to win their freedom.

The Iraqi statement today said that keeping foreigners in Iraq could be "one of the means for achieving peace" because Bush and his allies would have to reconsider their policies. It urged families of those held in Iraq and Kuwait to pressure their governments to end the economic sanctions.

The statement read on Iraqi radio and television declared: "America and its allies have gone beyond an economic boycott which in essence means only a non-exchange of goods. They embarked on the implementation of an economic blockade by force of arms against Iraq, including food and medicines, and this is an act of war under world norms and international law."

The Iraqi News Agency said today that Egyptians were exempted from the decision to move foreigners to key installations to deter attacks. At the outset of the conflict, Egypt had 1.6 million contract workers in Iraq and another 150,000 in Kuwait, although thousands have fled overland through Jordan in the past two weeks.

"Egyptians are sons of the Arab nation, and when they live in Iraq they are amongst their families and relatives and are welcomed by Iraqis with all warmth," the news agency declared.

Egypt has sent several thousand soldiers to Saudi Arabia to help Western troops and an all-Arab deterrent force protect Saudi air bases and oil installations from possible attack by the estimated 150,000 Iraqi troops massed in Kuwait along the border with Saudi Arabia.

Saddam and the official Iraqi media have maintained relentless verbal attacks against Mubarak for participating in the military deployment in Saudi Arabia, calling him an "imperialist agent" and urging the Moslem masses in Egypt to overthrow him.