BEIRUT, Lebanon (AP) _ Syrian shellfire sank a Greek-owned tanker that delivered oil to besieged Christians on Monday, and aides said army commander Michel Aoun has decided to order air strikes to break Syria's sea blockade.

Christian Gen. Michel Aoun refused to specify his plans Monday. But, when asked if a naval confrontation was imminent, he said: ''Everything is possible and we shall use the means in our posession.''

Aoun decided on the bombing runs against U.S. advice that he avoid escalating hostilities and undermining Arab League peacemaking efforts, his aides said on condition of anonymity.

From the bunker of his shell-ravaged presidential palace, Aoun told reporters: ''We have no aggressive intentions. But the Syrians are escalating their acts of piracy in our waters and in international waters.''

An Aoun aide said that after four months of the blockade, the general ''can't take the Syrian naval siege anymore. He's going to try to break it by force, using the air force.''

Aoun has three old British-made Hawker Hunter jet fighters and eight French-made Gazelle helicopter gunships in operation.

His aides say Syria has six gunboats patrolling offshore, as well as sophisticated MiG-23 fighter-bombers. Syrian jets in the past have been shot down by Israeli warplanes, which patrol Lebanese airspace and claim Lebanese skies are within Israel's sphere of influence.

Aoun's decision was reported as gunners from the Syrian Army and Aoun's mainly Christian Lebanese Army dueled with howitzers and rockets, and street gunbattles between rival Shiite Moslems ebbed into sporadic sniping.

In south Lebanon, Palestinian and leftist Lebanese guerrillas attacked Israeli troops patrolling with allied Lebanese militiamen. Three guerrillas were killed in the shootout, police said.

Aoun's aides said the 54-year-old general decided to use the air force against the Syrian navy last week, when gunboats were deployed to tighten the four-month-old blockade.

They said Aoun communicated his decision to ambassadors of the five permanent members of the U.N. Security Council: Britain, China, France, the Soviet Union and the United States.

U.S. Ambassador John McCarthy, however, prevailed upon Aoun to postpone the strikes, according to aides.

''But with Syrian gunboats now intercepting various ships heading for the Christian coast almost daily, the general is brushing aside the American ambassador's advice,'' the aide said.

Gunboats have intercepted a fuel tanker and three cargo ships carrying food in the past two days and taken them to Lebanon's Syrian-controlled port of Tripoli, Aoun's aides reported.

A Lebanese yacht with six men aboard was seized by two Syrian gunboats before dawn Sunday after it sailed from Kaslik Christian harbor. It was escorted to Syria's northern Tartus naval base, they said.

Police said Syria attacked the Maltese-registered tanker, Aloil, after it unloaded its cargo of gasoline at the Christian port of Jounieh. All eight crewmen were rescued.

The London-based Lloyd's Shipping Intelligence Unit, which monitors maritime activity, said the Aloil sank. It said the vessel is owned by Alco Shipping Co. of Piraeus, Greece.

At least five other cargo vessels have been hit by Syrian shellfire since the siege was imposed March 14. A Turkish sailor was killed and an Indian sailor was missing and presumed dead.

Also Monday, a Syrian gunboat intercepted the Santa Maria hovercraft that has been running the gauntlet of Syrian shellfire to ferry besieged Christians to Cyprus. The ferry, en route from Jounieh to Larnaca, was held for two hours.

Syria's military command did not comment on Aoun's reported decision. About 40,000 Syrian troops are stationed in Lebanon. They have violated a 1976 Arab League mandate by siding with Moslems against the Christians, who have 20,000 troops.

Two people were killed and five wounded in the overnight exchange of shellfire in Beirut and north Lebanon, police said. This raises the toll to 405 killed and 1,581 injured since the Syrian-Christian confrontation broke out March 8.

Syrian troops who police west Beirut were deployed Monday in Ouzai and Jnah residential districts to try to disengage warring militiamen of the pro-Syrian mainstream Shiite Amal and Iranian-backed fundamentalist Hezbollah, or Party of God.

The troops restored traffic between Beirut and south Lebanon.

Amal and Hezbollah are vying to dominate Lebanon's 1.2 million Shiites, the country's largest sect.