The Lebanese government's top security body held an emergency meeting and the Shiite militant group Hezbollah put its fighters on high alert.
Lebanon and Syria share a complicated history and a web of political and sectarian ties and rivalries. The uprising against President Bashar Assad has intensified divisions among Lebanese religious groups as well as polarization among those who support him and those backing the rebels fighting to topple him.
Lebanon has become completely consumed by the civil war next door. Car bombings, rockets, kidnappings and sectarian clashes — all related to the conflict — have become increasingly common in recent months.
Hezbollah, a staunch ally of the Syrian regime, has sent its fighters to back Assad's forces against the rebels and the militant group's leader, Sheikh Hassan Nasrallah, has suggested he would to do everything it takes to save the regime.
Adding to the jitters, the U.S. said it had instructed its nonessential staff to leave Beirut and urged private American citizens to get out of Lebanon.
The step had been under consideration since last week, when President Barack Obama said he was contemplating military action against the Syrian government for its alleged chemical weapons attack last month that killed hundreds near Damascus.
"Lebanese government authorities are not able to guarantee protection for citizens or visitors to the country should violence erupt suddenly. Access to borders, airports, roads, and seaports can be interrupted with little or no warning," a State Department statement said.
Shortly after the announcement, about 150 people drawn from several pro-Syrian political groups gathered for a peaceful protest near the U.S. Embassy compound north of Beirut, pledging larger rallies in case of a U.S. attack in Syria. Some of them had painted their hands red, symbolizing blood.
"The American Embassy is an operations room for the war on Syria," read one banner. "Your rockets and fleets do not scare us," read another.
Dozens of riot police in full gear stood on guard, confining the protesters to a square on a road leading to the heavily fortified embassy in the suburb of Awkar. The small protest reflected the potential for a surge in violence in case of military action, not just in Lebanon but across the region.
Iranian-backed Shiite militias in Iraq threatened to retaliate against Washington's interests inside Iraq if the U.S. goes ahead with strikes against Syria, according to Iraqi security officials and militants themselves.
Cleric Wathiq al-Batat, who leads the Mukhtar Army, a shadowy Iranian-backed militia in Iraq, said his forces are preparing for a strong reaction against the interests of the U.S. and other countries that take part in any Syria strike. He claimed that militants have selected hundreds of potential targets, which could include both official American sites and companies "associated with the Americans."
"There is a good level of coordination with Iran on this issue and I cannot reveal more. But I can say that there will be a strong response," he told The Associated Press. "Each armed group will have duties to carry out."
The worst-case scenario for Lebanon would be if Hezbollah, also a close ally of Iran, were to retaliate by launching a barrage of rockets at U.S. ally Israel. Such a move would be almost certain to trigger a response by Israel that would leave a trail of destruction in its wake.
The group has mobilized its fighters in southern Lebanon, redeploying and putting thousands of its fighters on high alert, according to residents and a Lebanese security official.
In villages close to the border with Israel, Hezbollah operatives were taking extra precautionary measures, and some have switched off their mobile phones to avoid detection, one official close to the group told AP. The officials spoke on condition of anonymity because they were not authorized to give statements to journalists.
Analysts, however, say the Shiite group, which fought Israel to a draw in a 34-day war in 2006 that destroyed much of southern Lebanon, is wary of picking such a fight right now on its own.
"I don't think Hezbollah will open the front from south Lebanon if there is no war between Israel and Syria," said Hisham Jaber, a retired Lebanese army general and director of the Middle East Center for Studies and Political Research in Beirut.
"Hezbollah will not play that very dangerous game, which would bring the whole area to a very, very dangerous war."
The 2006 war, which began abruptly after Hezbollah kidnapped Israeli soldiers in a daring cross-border attack, triggered a massive evacuation effort by foreign embassies of their nationals from Lebanon, including about 15,000 Americans who were removed by air and sea after Israel bombed runways and fuel tankers at Beirut's airport.
The likelihood of Hezbollah provoking Israel into a fight depends largely on the nature of any U.S.-led strikes on Syria. Prolonged action that threatens the regime may compel the group to act in a last-ditch effort to save its main backer.
Analysts say the group may send previously unknown groups to fire rockets across the border into Israel or to take Western hostages, while avoiding taking responsibility for such action. There are also a host of radical Palestinian and other militant groups in Lebanon, and there are concerns they may target Western interests.
"In Lebanon, anybody can do anything and get away with it," said Hilal Khashan, a political science professor at American University of Beirut. "These groups can do things with impunity in this country, and that's why it may not be advisable for Western diplomats to hang on for the forthcoming period."
Lebanon's Higher Defense Council, the country's top security body, met Friday to discuss measures to protect diplomatic missions and deal with a possible new wave of refugees fleeing Syria to Lebanon. The country of 4.5 million is already hosting nearly 1 million Syrian refugees.
Hezbollah's involvement in Syria has contributed largely to sectarian tensions in Lebanon, with Sunni Muslims mostly backing the overwhelmingly Sunni Syrian rebels. Many Shiites support Assad, a member of Syria's minority Alawite sect, an offshoot of Shiite Islam.
The State Department announcement came as David Hale, the new U.S. ambassador, presented his credentials to Lebanese President Michel Suleiman.
He said the U.S. is "very focused on insulating Lebanon from any aftermath of any response to Syria's chemical attack."
He blasted Hezbollah for its blatant involvement in Syria, accusing the group of "exacerbating the challenges to Lebanon."
In Syria, the government sent reinforcements including tanks and armored personnel carriers to the predominantly Christian village of Maaloula north of Damascus, where rebels battled regime troops this week.
The government forces sent to Maaloula have taken up positions outside the ancient village, which is still under the control of local pro-regime militias, activists said. There were skirmishes Friday around the village, home to two of the oldest surviving monasteries in Syria — Mar Sarkis and Mar Takla.
Associated Press writers Qassim Abdul-Zahra and Adam Schreck in Baghdad, and Hussein Malla in Awkar, Lebanon, contributed to this report.
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