Syrian-supported Palestinian commandos gained more ground today at the narrow security belt in southern Lebanon that rightist Christians have been building for Israel over the past six months.

White at least a dozen Israeli jets swooped over the battlefield, Palestinians took control of the villiage of Khiam, allowing them to tighten further their noose around the key crossroads town of Marjayoun.

Palestinian commandos said tonight they have taken half of Marjayoun, which controls access from the south to the "Fatahland," the rock slopes of Mt. Hermon from which Palestinians used to stage raids over the Lebanese border into Israel.

The Palestinians reported that Christians were shelling from Khiam tonight and that they expected a counterattack in the morning.

Diplomats here are concerned that the intensification of the fighting in south Lebanon this week - including the strong Syrian support for the Palestinians - could escalate into a full scale Arab-Israeli war.

The Syrians entered the fighting in the south for the first time Monday, using heavy artillery and 140 mm rockets - weapons that no Palestinians have - to shell Marjayoun. Now the fighting is being shared by Fatah, the strongest and best-trained Palestinian commando group, and Saiqa, a Palestinian group controlled Syria.

So far the Israeli reaction has been restrained. Its jets did nothing more than observe the battle and, according to sources here, take aerial photographs of Palestinian positions.

Official Lebanese government officials in the southern port city of Sidon said 12 Israeli-supplied armored peronnel carriers today rolled into the town of Air Eble, just stouth of the Palestinian today from going to Ain Ebel.

The Christian warlords, on the defensive for the first time since they started their campaign to clear Palestinians and Moslems from villages along Lebanon's border with Israel, today appealed to Arab nations to curb the Palestinian Guerillas.

In notes handed to all Arab embassies here, the Christian leaders said the fighting in the south threatened to rekindle the 19-month Lebanese civil war, which was ended in November by the arrival here of a 30,000 man Syrian-dominated Arab peace force.

The south is the only area of the country where Syrian forces have not established firm control.

When Syria moved a small force of 600 men and 10 tanks to the southern town of Nabatiyeh in February in an attempt to control Palestinians in the area, Israel reinforced its border an raised such a diplomatic furor that Syria was forced to withdraw.

Now Nabatiyeh has been turned into an armed Palestinian camp, the launching point for the assault on Israeli-supported strongpoints to the southe. Eyewithnesses there said Saiqa during the past 24 hours has moved heavy artillery and rockets there including rockets whose range is great enough to shell Israeli border settlements.

Saiqa officials said tonight they supplied the bulk of the Palestinian fire power for the asault on Khiam and Marjayoun. They said 500 of their commands took part in the storming of the villages.

I all, sources here said, 2,500 palestinian and leftist Moslem fighters are in the south Lebanon battle area with another 1,500 behind them in reserve.

They are facing about 1,200 Christian troops, some of them members of the regular Christian army and others sent from the Christian section in north Lebanon by sea to Israel.

The loss of Khiam was a heavy blow to the Christians, who won control of it in December, since it makes it easier for the Palestians to attack Marjayoun and Qlaia, another key garrison town.