(CNN) -- The international community called for restraint Monday after deadly clashes were triggered in Beirut by the deaths of two religious clerics, both of whom were opposed to Syrian President Bashar al-Assad.
Two other people were killed and 18 were wounded in the Lebanese capital early Monday as clashes flared between rival political parties -- one supporting al-Assad and the other opposing him, the country's National News Agency said.
The violence followed the killings of the two clerics, who were shot just hours earlier at a military checkpoint in northern Lebanon.
Derek Plumbly, the United Nations' special coordinator for Lebanon, called for an end to the violence and said the incidents must be "thoroughly investigated." And U.S. State Department spokesman Mark Toner said the United States is "concerned by the security situation."
"We call on all parties to exercise restraint and respect for Lebanon's security and stability," Toner said in a statement.
France similarly condemned the killings and praised the swift response by Lebanese authorities.
"Given the tense situation, we urge all parties to refrain from provocation," a statement from the French Foreign Ministry said. "Tensions can only be eased through dialogue."
Syrian troops were deployed in Lebanon between 1976 and 2005, primarily in the north. They were initially called in to help stop a brewing civil war, but maintained their significant presence -- which once numbered 40,000 -- long afterward.
In a country struggling to maintain a delicate balance among its religious and ethnic sects, resentment from the occupation lingers.
Some Sunni Muslims are staunchly against al-Assad and sympathize with the Sunni-led uprising in Syria calling for his ouster. Support for al-Assad is also plentiful, particularly in the south.
In recent weeks, as the Assad regime has shown no mercy in its crackdown on anti-government protesters inside Syria, the simmering animosity has boiled over.
Factions supporting and opposing the uprising in Syria clashed last week in the northern Lebanese city of Tripoli. Casualty counts vary, with some reports placing the death toll as high as eight.
On Friday, a video surfaced on YouTube that purports to show Lebanon's military arresting a Syrian activist at a hospital in Tripoli where he was being treated for wounds he suffered when Lebanese forces attempted to break up clashes between pro- and anti-Syrian regime groups.
And on Sunday, the two Sunni Muslim clerics were shot and killed at a checkpoint in the northern state of Akkar.
The National News Agency said soldiers fired on the clerics' convoy after it did not "abide by the Army's instruction" and stop at the checkpoint on its way to a sit-in organized by the anti-Assad Movement of the Future party.
The army immediately issued a statement of regret and said it had opened an investigation.
But Saad Hariri, a former Lebanese prime minister and leader of the Movement of the Future party, pointed the finger at al-Assad "infiltrators" in the military for the deaths.
"There are some infiltrators who want to use the military and its image to import the crisis of the Syrian regime to Lebanon in a desperate attempt to save the Assad regime from its inevitable end," he said.
Abdel Qadir Abdel Wahid, the brother of one the slain clerics, echoed the sentiments in an interview with MTV Lebanon.
"Unfortunately, we have some infiltrated elements in the Lebanese army, possibly officers who work for the Assad regime," he said. "I hope that the army institution cleanses itself from these elements."
Hours later, armed clashes erupted in Beirut between Future supporters and those of the pro-Assad Arab Movement. In addition to the deaths, the clashes left shops damaged and cars torched, the national news agency said.
Plumbly, in a statement Monday, said, "Since I arrived in Lebanon, I have been impressed by the efforts of the security authorities and political leaders to safeguard Lebanon's calm and stability, at a time of upheaval and uncertainty in the region. I am confident now that all parties in Lebanon will continue to put the interests of the country above other considerations. Differences must be addressed through dialogue," not violence.
The fighting has prompted Kuwait, Qatar and the United Arab Emirates to advise their citizens not to travel to Lebanon.
"The civil war in Syria is likely to prove a watershed for the Middle East's balance of power," said Hillary Mann Leverett, who teaches foreign policy at American University in Washington. "It has the potential to become a full-blown regional war that could spill over into other countries and bring those countries into conflict, through proxies and perhaps even directly."
Al-Assad, in a rare interview last week, put the blame for the Syria violence on the so-called Arab Spring, during which popular revolutions have toppled the governments of Egypt, Libya and Tunisia. He also alleged that weapons bound for rebels were entering his country from Lebanon.
"For the leaders of these countries, it's becoming clear that this is not 'Spring' but chaos," he told Rossiya 24. "And as I have said, if you sow chaos in Syria, you may be infected by it yourself, and they understand this perfectly well."
CNN's Saad Abedine contributed to this report.