The Syrian Army moved to pressure rebellious Christian militiamen threatening the government of President Amin Gemayel today, raising fears of a confrontation, and another 11 members of the U.S. Embassy staff in Beirut were evacuated because of the deteriorating security situation.

Further underscoring these security concerns, three armed gunmen kidnaped the American bureau chief of The Associated Press in Beirut earlier today.

The Syrian Army cut off the coastal highway from Tripoli to Beirut at the Madfoun Bridge, 29 miles north of Beirut. It also was reported to have deployed a brigade of 25 tanks and eight mounted rocket launchers near the bridge tonight, posing a potential confrontation between the Syrians, who are backing the "national unity" government led by Gemayel, and the breakaway Lebanese Forces led by rebel commander Samir Geagea.

Geagea is trying to reverse Syrian influence in an attempt to negotiate a political settlement to sectarian fighting and oust Gemayel from the leadership of the mainstream Christian-based Phalangist Party.

Amid continuing reports of threats against Americans in Lebanon, Terry A. Anderson, the AP bureau chief here for more than two years, was forced into the gunmen's car as an AP photographer, Donald Mell, was held at bay at gunpoint. Anderson was the third foreigner kidnaped in predominantly Moslem west Beirut during the past three days. No group claimed responsibility for the abduction.

As they did last Thursday after evacuating 13 embassy employes to Cyprus, U.S. officials characterized the airlift as a "temporary measure" stemming from threats of terrorism and the uncertain security situation in east Beirut resulting from clashes between rival Christian militiamen in that predominantly Christian sector of the capital.

In Washington, the State Department issued a statement saying the U.S. Embassy in Beirut "continues to function" and characterizing the removal of Americans as "prudent, temporary reductions in staff consistent with prevailing security conditions" and "not an evacuation." It also deplored Anderson's abduction and reiterated advice to Americans in Beirut "to take advantages of the opportunities to leave."

A senior source in the Lebanese Forces command in Beirut said tonight that Geagea's militia was "observing for the moment" to determine the Syrians' intention. Asked whether Lebanese Forces reinforcements would be moved from Jounieh and other Christian strongholds to the Barbarah checkpoint about four miles south of the Syrian Army position, the source said, "It depends on their the Syrians' intentions. If it appears that they have intentions to move, we are prepared to confront them."

The militia official said, however, that he regards the troop and armored vehicle movement as "some kind of warning with regard to a red line, to show off muscles; some kind of pressure, either economic, political or military."

The economic pressure, he said, would stem from cutting off the highway to prevent Christians living in northern Lebanon from reaching Jounieh and Christian east Beirut.

Just before dark today, there was evidence of a Syrian buildup just north of the Madfoun Bridge, but no sign of the tank brigade or rocket launchers described by the Lebanese Forces official. However, the official said that a top Lebanese Forces commander, Fuad Abu Nader, received word of the Syrian Army redeployment at 6:15 p.m., after darkness and about 1 1/2 hours after two correspondents drove through the Syrian roadblock for about a mile and saw only small contingents of Army commandos. In a field near the bridge, a small Army unit was setting up a mortar battery. The mortars were pointed toward the Christian militiamen at the checkpoint to the south.

The state-run television said tonight that in addition to the tank brigade, the Syrian buildup included about 20 armored personnel carriers and heavy machine-gun emplacements in a line just north of the Madfoun Bridge.

Syrian Defense Minister Mustafa Tlas, according to news agency reports from Damascus, confirmed the presence of the tank brigade, saying that "the objective of this move is to assist President Amin Gemayel and the Lebanese legitimacy." The term "Lebanese legitimacy" is used to describe the Syrian-backed national unity government.

Although the Syrian Army moved only about a mile from its previous position north of the Madfoun Bridge, the move to the fringes of the Christian heartland symbolized Syria's determination not to permit the Geagea-led Lebanese Forces rebellion to scuttle attempts by Damascus to apportion parliamentary representation and government patronage between Moslems and Christians in a way aimed at ending years of sectarian violence in Lebanon.

Although it only has brigade strength deployed against the Christian militias at Madfoun now, Syria could muster a force that could intimidate Geagea's militia backers, although militiamen interviewed in a barracks and at a checkpoint near the bridge gave no indication of being intimidated.

One militiaman, asked if he was frightened by the Syrian buildup, answered, "If they move, we will welcome them." His sardonic smile indicated a readiness for battle. Other militiamen excitedly asked the correspondents who had driven through the Syrian positions whether they thought an attack or a drive south toward east Beirut was likely.

The increase in reported threats against Americans in Beirut followed the U.S. veto Monday of a U.N. Security Council resolution that would have condemned Israeli security practices in the portion of southern Lebanon that it still occupies. Gunmen kidnaped a British businessman, Brian Levick, near his home on Friday, the day after British metallurgist Geoffrey Nash was abducted while walking to work.

The British ambassador, David Miers, advised all British nationals to leave Beirut unless they have "compelling reasons" to stay. British Embassy officials suggested today that both of the kidnaped men may have been mistaken for Americans.

There was no immediate claim of responsibility for any of the three abductions. Fundamentalist Shiite Moslem groups have claimed responsibility for other kidnapings and terror bombings, such as Islamic Jihad, a shadowy organization that claims it is holding four other Americans still missing after being abducted during the last 12 months.

AP photographer Mell said the kidnaping of Anderson occurred shortly after 8 a.m. near the seafront Corniche Mazraa when he was being let off by Anderson at his home after the two had played tennis. The kidnapers apparently had watched the movements of the two in the neighborhood, where a number of foreign journalists maintain apartments.

Mell said he was held at gunpoint while the kidnapers forced Anderson into a car and drove away.

Another American journalist, Jeremy Levin, then the Beirut bureau chief for Cable News Network, was kidnaped last year and held in a house in eastern Lebanon's Bekaa Valley for 11 months before, he said, he escaped last month and walked to a Syrian Army division.

Kidnaped Americans still missing are Presbyterian minister Benjamin Weir, U.S. Embassy political officer William Buckley, university librarian Peter Kilburn and Roman Catholic priest Lawrence Jenco.