At least 17 persons were killed and 80 injured last night when a suspected bomb blast leveled a crowded restaurant popular with U.S. servicemen from an air base outside Madrid, officials said. Two of the dead and 12 of the injured were believed to be Americans. The Madrid city governor, police and a witness said that they believed that a bomb had caused the explosion, which occurred shortly before midnight, when the restaurant was packed with 300 diners. The blast caused the collapse of the three-story building housing the restaurant. A Madrid city government official said he believed the bomb had been made up of 44 pounds of plastic explosives. Although it was unclear whether the explosion was caused by a terrorist bomb, the blast raised fears of renewed violence against U.S. and NATO targets in Western Europe. Madrid's Antena 3 radio and two Basque country newspapers said they had received calls in which purported members of the Basque separatist guerrilla group ETA claimed responsibility for the explosion. But the radio later said a police anti-guerrilla expert had examined a recording of the call and pronounced it false, and observers here tended to discount the theory that Basque separatists might have bombed the restaurant. There were suspicions, however, that leftist guerrillas might have targeted the establishment because of its popularity with U.S. personnel. Spanish National Radio said it had received unconfirmed reports that two of the dead and 12 of those injured were Americans from the nearby Torrejon Air Base, one of four Spanish military bases where about 12,000 U.S. troops are stationed. A U.S. military ambulance sped to the El Descanso restaurant, and American military rescue personnel were among those searching through the rubble of the building. Officials said 10 slightly injured Americans had been taken to the hospital at the Torrejon base. A duty officer at Madrid police headquarters said he could not confirm reports on the blast's cause until bomb squard experts completed their work, but he added: "Judging by the characteristics of the explosion - the gas pipes were intact, for instance - it looks like a bomb." Madrid's civil governor, Jose Maria Rodriguez Colorado, also said it could have been a bomb. Asked by a local radio station whether it was a guerrilla bomb attack, he said: "I still don't dare to confirm that it was an action of this kind, but there are possibilities that it was so." Urban guerrillas recently have hit NATO installations and personnel with a spate of bomb and gun attacks in Belgium, West Germany, France, Portugal, Spain, Greece and the Netherlands in what newspapers have called "Euroterrorism." West Germany's Red Army Faction and France's Direct Action urban guerrilla groups announced in January that they were joining forces to fight NATO. Direct Action is known to have ties with another leftist group, Belgium's Fighting Communist Cells, which started a bombing campaign against NATO fuel pipelines and other installations last year. All three groups oppose the deployment of U.S. missiles in Europe as well as the NATO alliance itself. Earlier this year, the Red Army Faction killed an arms industrialist, and French urban guerrillas shot dead a general in charge of French arms sales. But NATO experts believe that, with the exception of the two slayings, the recent attacks on NATO targets in West Europe have not been highly coordinated. One official was quoted in February as calling it "me-too terrorism." Police said at least one of the victims of today's blast, a black woman, was foreign, but her identity was not known. The barbeque restaurant, El Descanso, is on the road to Barajas Airport and nine miles from downtown Madrid. U.S. personnel from the Torrejon base made up about 25 percent of the regular customers at the restaurant, located on the ground floor of a building. Juan Jose Gonzalez, the son of the restaurant's owner, said he saw a bomb go off. He said he saw a flash and then was hit by the shock wave of an explosion that blew up the door of the restroom. Asked whether he was sure it had been a bomb, he replied, "I don't have the slightest doubt that it was." He added that he had "collapsed from the shock wave and was completely buried by debris." Several hours after the explosion, new victims were being rescued from the rubble by firefighters, Red Cross workers, police and U.S. military rescue personnel. Callers to a radio station said ETA and its ally, the urban guerrilla group GRAPO, had timed the bomb to go off today at 8:45 a.m. local time, when U.S. servicemen have their breakfast, but the device exploded by mistake at dinner time. A recording of the call broadcast by a Madrid radio station said: "The bomb was meant as an attack against the Yankee armed forces." The called added: "It was a mistake. We apologize to the victims. We are very sorry."