Lebanese troops launched a major security operation on Monday to open all roads and force gunmen off the streets, trying to contain an outburst of violence set off by the assassination of a top intelligence official who was a powerful opponent of Syria.

Sectarian clashes overnight killed at least five people in Beirut and Tripoli.

Lebanese Army soldiers patrol a neighborhood after overnight clashes between Sunni and Shia gunmen in Beirut, Lebanon, on Oct. 22, 2012. (Ahmad Omar/Associated Press)

Opponents of Syria have blamed the regime in Damascus for the killing of Lebanese Brig. Gen. Wissam al-Hassan in a Beirut car bomb on Friday.

With Lebanon already tense and deeply divided over the civil war next door, the assassination has threatened to drag the country back into the kind of sectarian strife that plagued it for decades — much of it linked to Syria.

"The nation is passing through a crucial and critical period and tension has risen in some areas to unprecedented levels," the army said in a statement. It urged politicians to be careful not to incite violence "because the fate of the nation is on the edge."

"Security is a red line," the statement said, adding that strict measures are being taken to "prevent Lebanon from being an arena for settling regional problems."

'The fate of the nation is on the edge.'— Lebanese Army statement

Sporadic cracks of gunfire rang out in Beirut as soldiers backed by armoured personal carriers with heavy machine-guns took up position on major thoroughfares and dismantled roadblocks. At times, troops exchanged gunfire with Sunni gunmen.

The state news agency reported sporadic gunfire in parts of Beirut and around the northern city of Tripoli.

Tripoli saw clashes between two neighbourhoods that support opposite sides in Syria's conflict and have a decades-long history of shooting at each other. Four people were killed in the fighting between the Sunni neighbourhood of Bab Tabbaneh and the adjacent Alawite neighbourhood of Jabal Mohsen, which supports the Syrian regime.

At least 17 wounded

Tripoli residents said scores of soldiers deployed around the city in an attempt to bring back calm. The military also set up checkpoints, searched cars and asked people for identity cards. Security officials also said one man was killed in the Wadi Zayneh area north of the southern city of Sidon.

They said the clashes also wounded at least six people in Beirut and 11 in Tripoli. The officials spoke on condition of anonymity because they were not authorized to speak to the media.

An Associated Press photographer saw dozens of gunmen roaming the streets on Monday in Beirut's predominantly Sunni neighbourhood of Tariq Jadideh, where fighting has taken place. Local Sunni leaders were calling the gunmen by telephone urging them to pull out of the streets.

In some roads around Tariq Jadideh, masked Sunni gunmen set up checkpoints, stopping cars and asking people about their destination and where they were coming from.

A woman who lives in the neighbourhood said the fighting began shortly after midnight and lasted until sunrise.

"We couldn't sleep because of the shooting. There were also some booms," she said, referring to rocket-propelled grenades. She asked that her name not be used for fear of reprisals.

Jordanian soldier killed in Syria-related violence

Meanwhile, a Jordanian soldier was killed in clashes with armed militants trying to cross the border into Syria on Monday.

Jordanian Information Minister Sameeh Maaytah said the soldier was the first killed in violence related to Syria's civil war. He died in clashes with militants trying to illegally enter Syria to join rebels fighting Assad's regime. Maaytah did not say whether the militants were Jordanians or foreign fighters trying to jump into the fray in the neighboring country.

A number of foreign Islamists have been fighting in Syria alongside the rebels. Jordan's banned Salafi movement — which promotes an ultraconservative brand of Islam — has sent several fighters to Syria in past months and Jordanian border patrols have caught some of them recently.

Lebanon and Syria share similar sectarian divides that have fed tensions in both countries, increasingly. Most of Lebanon's Sunnis have backed Syria's mainly Sunni rebels, while Lebanese Shias tend to back Assad.

Al-Hassan was a Sunni who challenged Syria and its powerful Lebanese ally, the Shia militant group Hezbollah. The uprising in Syria is dominated by the Sunni majority fighting Syrian President Bashar al-Assad, who like many who dominate his regime, is a member of the Alawite sect — an offshoot of Shia Islam.

Al-Hassan's assassination has imperiled Lebanon's fragile political balance. Many politicians blamed Syria for the killing and angry protesters tried to storm the government palace after al-Hassan's funeral on Sunday, venting their rage at leaders they consider puppets of a murderous Syrian regime. But they were pushed back by troops who opened fire in the air and fired tear gas.

UN expresses support for Lebanon

Prime Minister Najib Mikati, a Sunni, told As-Safir newspaper that when he took up his post last year, he intended to protect all Lebanese, particularly Sunnis.

"I was convinced that through this mission, I am protecting my country, my people and especially fellow members of my sect."

The prime minister of Lebanon is usually a Sunni according to a sectarian division of top posts in the state. Over the past year, pro-Syrian Hezbollah and its allies have come to dominate the government.

On Sunday night, a group of anti-Syrian protesters started an open-ended sit-in outside Mikati's house in his hometown of Tripoli. The protesters said they will only end the sit-in when Mikati resigns.

Ambassadors of Britain, the U.S., Russia, China and France and the UN Special Coordinator for Lebanon met President Michel Suleiman to express support for him.

"The permanent members at the United Nations call upon all the parties in Lebanon to preserve stability," Derek Plumbly, the UN representative, told reporters in Arabic while surrounded by the five ambassadors. "We strongly condemn any attempt to shake Lebanon's stability."

Later in the day, Mikati met with Suleiman but did not make any statements afterward.