PARIS, SEPT. 14 -- Iraqi soldiers in Kuwait stormed the diplomatic compounds of France, Canada and Belgium early today, kidnapping four French citizens in a move that prompted international condemnation and vows of retaliation from the French government.

A U.S. diplomat was inside the Canadian ambassador's residence when Iraqi troops entered and was briefly detained along with other Western diplomats. President Bush, at a news conference in Washington, condemned the detentions and the Iraqi raids, saying they were of "grave concern" to the United States and vowing to support French President Francois Mitterrand in "whatever he decided to do" in retaliation. {Details, Page A18}

In another sign of escalating tension in the Persian Gulf crisis, a U.S. warship fired a warning shot at an Iraqi vessel in the Gulf of Oman and then boarded it to search for contraband, with help from an Australian frigate. U.S. warships had fired warning shots in two previous incidents but officials said this was the first such boarding of an Iraqi ship. The vessel had no banned cargo and was allowed to proceed, a Navy spokesman said.

Nations opposed to Iraqi President Saddam Hussein moved today to support the U.S.-led mobilization in the gulf with new pledges of troops, warplanes and financial aid. Britain became the first European country to commit substantial ground forces to the region, where at least 150,000 U.S. troops are deployed.

Syria promised to send more troops and France hinted it would do the same, while Canada and Italy each pledged a squadron of jet fighters. Japan, stung by criticism that it has not done enough in the crisis, said it was contributing $3 billion beyond the more than $1 billion it had pledged earlier to help pay the operation's staggering cost and provide economic aid to countries severely affected by the U.N.-mandated sanctions against Iraq.

The embassy intrusions, which began Thursday when Iraqi forces entered the Dutch ambassador's residence, represented the first instances that Iraqi troops have crossed the thresholds of Western diplomatic missions in Kuwait since Iraq annexed Kuwait as its 19th province after invading Aug. 2.

The French military attache, seized in a raid today, was released after several hours of detention, but the fate of the three other civilians, who had sought refuge inside the compound to avoid being taken hostage, was unknown.

President Francois Mitterrand, traveling on a visit to Czechoslovakia, scheduled an emergency cabinet session for Saturday morning. Mitterrand, who spoke by telephone with Bush shortly after hearing about the forced intrusion, said the action by Iraqi troops "was an aggression and we will respond to it."

"France has to take important decisions," Mitterrand declared, saying he would reveal them after consulting with ministers most closely involved in the crisis on Saturday. French Defense Minister Jean-Pierre Chevenement left for Saudi Arabia today and officials said he would be carrying a message from Mitterrand for King Fahd. Sources said the message was likely to include a new offer of French ground troops, tanks and aircraft to bolster Saudi defenses.

Another sign of Europe's tougher stand in the crisis came today as Britain ordered an armored brigade of 6,000 troops and 120 tanks to Saudi Arabia, and British Defense Secretary Tom King said a squadron of Tornado ground attack aircraft and additional air defense aircraft would also be sent.

Syrian President Hafez Assad told Secretary of State James A. Baker III during a four-hour meeting in Damascus today that he is willing to commit an armored division to the multinational force in the Persian Gulf, administration officials in the Syrian capital said. Syria has already deployed 3,000 troops in Saudi Arabia and 1,000 in the United Arab Emirates as part of the multinational force in the region.

Baker later flew to Rome, where he was informed by Italian Foreign Minister Gianni De Michelis that Italy will contribute a squadron of British-built Tornado jet fighters to the gulf deployment. The eight-plane squadron will require Italian maintenance troops on the ground.

Canada, too, announced deployments today, committing a squadron of CF-18 fighter jets to the gulf region to provide air cover for two Canadian destroyers already en route. "This is not a conflict between Arabs and the West, nor is it a conflict between Iraq and the United States," said Canadian Prime Minister Brian Mulroney. "It is Saddam Hussein against the civilized world."

The United Nations Security Council sent a new signal that it was maintaining pressure on Saddam, voting late Thursday night to impose strict controls on humanitarian food aid to Iraq and Kuwait. The move was an attempt to stop supporters of Iraq from breaking the trade embargo by sending food and other supplies under the guise of humanitarian aid.

In addition to France, Canada and Belgium also reported that their diplomatic sanctuaries were violated by Iraqi troops today. Soldiers entered the Canadian ambassador's residence before dawn and briefly detained the consul along with diplomats from the United States, Ireland and Australia who were meeting with him at the time to plan a further evacuation of their nationals from Kuwait this weekend. The American was Joseph C. Wilson IV, the charge d'affaires in Baghdad, who was in Kuwait arranging refugee flights.

Iraqi troops also forced their way into the Belgian Embassy grounds and ordered the two remaining diplomats to leave, but the soldiers fled after the diplomats took shelter in the adjoining residence.

The incidents marked an escalation in the tactics employed by Iraqi troops since authorities in Baghdad demanded on Aug. 24 that diplomatic missions should be shut down and all personnel moved to the Iraqi capital because Kuwait was no longer a sovereign country.

Seventeen embassies have remained open in Kuwait in defiance of Iraq, despite efforts by Iraq to force them out. Water and electricity have been cut off and Iraqi troops have surrounded the embassies, but none had been entered by Iraqi troops until now. More than 30 countries have closed down operations because of the deteriorating situation there.

France maintains a staff of six diplomats in Kuwait, who have come under increasing harassment. Besides cutting off vital utilities, the Iraqis have knocked a hole in the wall of the French Embassy.

The official Iraqi News Agency denied that Iraqi soldiers had entered the foreign compounds, referred to as "former diplomatic missions." The official news agency said the troops were under strict orders not to enter the compounds.

The Iraqi raids, which appeared to violate the 1961 Vienna Convention on the inviolability of diplomatic missions, triggered immediate international condemnation. The North Atlantic Treaty Organization called Iraq's action "an intolerable breach of international law" and the 12-nation European Community demanded the "immediate release of the abducted foreigners."

In France, the raid met with universal criticism. The opposition Rally for the Republic urged Mitterrand to call for an emergency session of the U.N. Security Council to discuss measures to free the hostages.