Lebanon's rival Cabinets, teetering on the brink of a showdown, have closed Beirut's airport and harbors in tit-for-tat assertions of power.

The country's national carrier, Middle East Airlines (MEA), halted its flights to the Moslem-controlled airport after Christian Army Commander Maj. Gen. Michel Aoun ordered it closed Saturday.The Syrian-backed Cabinet of Selim Hoss retaliated Sunday by ordering the closure of the Christian-controlled ports of Beirut and Jounieh, north of the capital.

" . . . Sailing to the port of Beirut is not safe due to the security situation . . . We call on all maritime insurance companies to comply with the orders," said the Moslem Public Transport Ministry, headed by Druze militia leader Walid Jumblatt.

Political sources said the Moslem transport ministry has no power to enforce the closure of Beirut and Jounieh ports.

But Jumblatt's fighters can shell the Beirut harbor from their mountain strongholds.

Christians and Moslems have engaged in their worst fighting in almost two years after a period when the broader conflict took second place to battles within the two communities.

Jumblatt's men and army troops battled briefly Sunday with artillery, mortar and heavy machine-gun fire against troops at Souk al-Gharb, southeast of the capital.

Shells slammed into nearby mountain towns but there was no word on casualties.

The battles erupted after Aoun sent patrol boats last week to blockade seven Moslem militia ports south of Beirut.

Families along the Green Line dividing Beirut into Christian and Moslem sectors have begun sandbagging doors and windows in fear of fresh fighting there.

MEA said in a statement that it had halted its flights at Beirut because insurance companies had temporarily lifted their cover. It did not say when flights would resume.

The country's only civilian airfield, south of the capital, has been closed repeatedly, often for months at a time, in Lebanon's nearly 14 years of civil war.