Spanish authorities said today that an explosion Friday in a restaurant near Madrid frequented by U.S. servicemen appeared to have been the work of Islamic Jihad, the radical group that has claimed responsibility for several major terrorist attacks against U.S. and other western interests in the Middle East.

Interior Minister Jose Barrionuevo spoke after a long meeting with his security chiefs that included a review of different terrorist groups that allegedly had claimed responsibility for the explosion. Among the claims was one made to Reuter news agency in Beirut by Islamic Jihad.

"Police believe that the most credible claim was the one made in Beirut by the Shiite Moslem group Islamic Jihad," Barrionuevo said on Spanish radio. He gave no details.

The explosion killed 18 Spaniards and injured 15 Americans, one of them seriously. The bombed restaurant, El Descanso, allegedly was chosen as a target because it was frequented by many of the 4,500 U.S. personnel stationed at the Torrejon de Ardoz joint U.S.-Spanish air base.

Barrionuevo said the explosion had been caused by a homemade type of bomb weighing about 12 pounds and made of a chemical compound that included chloride, sulfur and potassium.

Islamic Jihad, which means "Islamic holy war," is a shadowy group believed to be an umbrella for radical Shiite Moslem terrorists based mostly in Lebanon. It is believed to be linked to Iranian, Syrian and Libyan interests. The group gained worldwide notoriety for its role in the suicide car-bomb attacks on the U.S. Embassy in Beirut and on American and French troops of the multinational force in Lebanon.

The group said in its claim that the Madrid bombing was the second attack carried out to avenge a car-bomb explosion in a Shiite suburb of Beirut on March 8 that killed 92 persons. At the time, Shiite groups blamed that bomb on Israel and the United States.

Security experts working on the Madrid bombing said they believe the mention of an earlier attack is a reference to a blast on March 29 in a Paris movie theater that was holding a Jewish film festival. Islamic Jihad was one of several groups that claimed responsibility.

The Beirut claim warned that there would be further "destruction" until the last Israeli soldier had left southern Lebanon.

In its first public statement on the blast, the U.S. Embassy in Madrid condemned it Sunday as a terrorist act and said it was a consequence of an international problem that all democracies should work together to fight, Reuter reported.

Some groups operating under the Islamic Jihad umbrella are said by security experts to maintain links with European terrorist organizations such as Direct Action in France, the Red Army Faction in West Germany and the Red Brigades in Italy. All of these groups have claimed responsibility in the past for attacks on U.S. and North Atlantic Treaty Organization targets in Europe.

Spanish security chiefs are investigating the possibility that domestic terrorist groups, notably the Basque separatist organization ETA and a leftist group known as GRAPO, could be part of the terrorist network.

One theory is that a Spanish group could have carried out the bombing at the behest of Islamic Jihad, although the type of explosive used Friday rarely has been used by Spanish terrorist groups.

Barrionuevo said the restaurant attack underlined the need for urgent, wide-ranging cooperation among the world's democracies to stamp out international terrorism. He said he would make this an urgent item on the agenda of a coming meeting of European interior ministers.