ATHENS, Greece (AP) _ A bomb exploded Saturday night in a suburban bar frequented by servicemen from a nearby U.S. Air Force facility, wounding four Americans and nine other people, police reported.

A police spokesman said the bomb went off at 11:10 p.m. in Oscar's bar near the U.S. base at Hellenikon Airport in suburban Glyfada.

''It apparently was a powerful device placed in a bag hanging from a coat- rack close to the bar counter,'' said the spokesman, who was not identified in line with police regulations.

No individual or group asserted responsibility for the bombing, according to police.

Two Greeks were admitted to the Asklipeion Voula Hospital with first- and second-degree burns, Dr. Ioannis Tsigalis told reporters.

He said five Greeks, two Britons and four American servicemen were treated for cuts and puncture wounds from flying glass and metal.

''We had to extract some fragments of metal embedded in cuts, but there were no serious injuries,'' Tsigalis said.

The police spokesman said the Americans were transferred to the hospital at the U.S. fase.

A hospital official, who spoke on condition of anonymity, gave the names of the Americans as Timothy Johnson, John Fines, Ricky Hoover and Larry Morrison, but he said he did not know their ages or hometowns.

There was no immediate comment from the American Embassy or officials at the Air Force base.

Witnesses said some 25 people were at the bar, which is decorated with American flags, when the bomb went off.

''I was standing at the bar having a drink when it blew up in front of me. There was a really loud explosion and I got dizzy and fell down,'' said a Greek patron, Costas Pabalos, 25.

The blast blew debris into the street and shattered windows in a nearby restaurant and apartment building.

''We heard a loud blast and saw people running out of Oscar's. There was a man running out of there screaming with his clothes on fire,'' said Dimitris Katsounis, the restaurant owner.

Oscar's is a noted meeting place for American servicemen from the base and other foreigners living in Glyfada, a seaside suburb 11 miles south of Athens.

More than 1,200 Americans are stationed at the 7206 Air Base Group at Hellenikon. Many live off base in the Glyfada and Voula suburbs.

American servicemen have been targeted by the left-wing guerrilla group calling itself November 17.

In April and August 1987, the group staged bomb attacks in Athens against buses carrying American military personnel. More than 30 Americans were injured in the attacks, one seriously.

In February 1985, a bomb exploded in Bobby's Bar in Glyfada and more than 30 Americans were wounded. No group clamed responsibility for that attack and no arrests have been made.

Saturday's attack occurred one week after Greek and American officials completed a fourth round of talks on a new agreement for the four U.S. military bases in Greece.

Premier Andreas Papandreou's socialist government has said a new bases agreement would be submitted for approval in a referendum before taking effect.

Many leftist groups oppose the U.S. military presence in Greece, a member of the North Atlantic Treaty Organization.