Christian militiamen in rebellion against the Lebanese government toned down their criticism of Syria today and said they had received assurances from Damascus that Syrian forces massing near areas held by the rebels would not attack.

The easing of the war-crisis atmosphere between the rebel Christians and Syrian forces came as Moslem radicals in west Beirut stepped up their campaign of intimidation of foreigners still living in the Lebanese capital and threatened to seize more hostages to be held as guarantees against reprisal raids by Israel or the United States.

[Two Israeli soldiers were killed and five wounded in an ambush Sunday in Jibsheet in southern Lebanon, Reuter reported from Tel Aviv.]

A caller claiming to represent Islamic Jihad, a shadowy organization believed to include several fundamentalist Shiite Moslem groups with close ties to Iran, said the group had carried out the kidnaping yesterday of Terry A. Anderson, the American bureau chief of The Associated Press, and two British businessmen.

The caller told news agencies the kidnapings were part of "our continuing operations against America and its agents." There was no way of determining the authenticity of the calls. Islamic Jihad has claimed responsibility for several previous kidnapings and bombings.

Moslem gunmen today seized a car and driver from the Italian Embassy and fired a grenade at the Chinese Embassy in apparent attempts to press a campaign to get foreign missions here to evacuate all non-Lebanese citizens. No casualties or serious damage were reported at the Chinese Embassy.

The signs of a lowering of tensions between Syria and the breakaway Lebanese Forces Christian militia unit led by Samir Geagea from his base in Jounieh, came at a press conference held by Karim Pakradouni. He is a ranking member of the "emergency committee" that the Christians formed last week after their militia declared its political and military independence from the Phalangist Party of Lebanese President Amin Gemayel.

Pakradouni, a close adviser to Gemayel's brother, Bashir, before his assassination as president-elect, said he had been in contact with top Syrian officials and was convinced that a military intervention to put down the militia's revolt against the Lebanese president was not imminent.

"The contacts so far completely prove that there will be no military influence by the Syrians in our region -- no invasion," Pakradouni said.

He said that he expected new contacts with Syrian Vice President Abdul Halim Khaddam within the next 24 hours, although no time was set. Khaddam is due here Monday for talks with Gemayel and other Lebanese leaders.

Fuad Abu Nader, commander-in-chief of the Lebanese Forces and a nephew of Gemayel, said in a separate press conference that Syrian troops, including one armored battalion, two commando battalions and one artillery battalion, had been placed on alert and moved south from their normal positions in northern Lebanon to the hills overlooking a Lebanese Forces checkpoint at the Madfoun Bridge just north of Barbarah, about 29 miles north of Beirut. He said the units included 2,000 to 3,000 men.

While Abu Nader said his own militia was preparing for a Syrian thrust south toward Beirut, he spoke in conciliatory tones, saying at one point, "Now, I think, this is the time for talking. Everybody has the will to talk."

He added, "We are feeling that there is no final [Syrian] decision on an intervention. But on the ground we are seeing the troops massed."

The closeness of Gemayel and his government to Syria has been a chief dispute between the two Christian factions. Supporters of Geagea have been closely allied to Israel and have demanded an end to Syria's involvement in Lebanon and the removal of its troops, which have been in the country since the 1975-76 civil war.

Another top-ranking member of the "emergency committee," who asked not to be identified, said efforts by the Lebanese Forces under Geagea to reduce the Syrian influence in an attempt to effect peace between warring sectarian factions in Lebanon had undoubtedly been noticed with alarm in Damascus.

"What has happened in our movement for sure has undermined the whole process of Syrianization of Lebanon, and the Syrians for sure are taking notice of it," the official said. "We believe political pressure will be put on us. The troop movements are being done for political pressure. But no direct involvement is expected."

Despite the increased buildup of Syrian troops, the Christian militia continued to maintain a low profile along the strategic coastal highway between Beirut and Tripoli, stationing only several lightly armed troops at their checkpoint at Barbarah.

The bulk of the Syrian force was kept out of sight of the bridge crossing, although infantry units positioned in the adjacent hills appeared edgy.

Journalists approaching the bridge in two cars this morning were fired upon by Syrian soldiers when the journalists, noticing that the road had been cut to all traffic, attempted to turn around and go back toward Jounieh.

As they headed away from the checkpoint, Syrian troops opened fire with long bursts from automatic rifles, hitting one of the vehicles several times and disabling it. No one was hurt.