Violence has flared in Lebanon after at least 10 of the army's officers and soldiers were killed in what the army has called a "cold-blooded attack".

Tensions rose after the army clashed with followers of a Sunni sheikh, Ahmad al-Assir, who is opposed to the Shia movement Hezbollah and its involvement in Syria's war.

"An armed group loyal to Sheikh Ahmad al-Assir attacked, for no reason, a Lebanese army checkpoint in the village of Abra" the Lebanese military said on Sunday. "Two officers and a soldier were killed, while several others were wounded."

Al Jazeera, however, understands that at least 10 military personnel were killed.

Assir's supporters had surrounded an army checkpoint in Abra, on the outskirts of Sidon, where a vehicle transporting other supporters of the Sunni cleric had been stopped, a security source said.

The army were fired on, and then returned fire, the source added.

For hours afterwards, intense battles engulfed the district, closing down businesses and forcing residents to flee with their children.

"The shells are raining down on us, and there is intense gunfire," one witness said.

Al Jazeera's Nour Samaha, reporting from Beirut, said tensions were rising elsewhere in Lebanon.

"They are heightened in other cities, most notably Tripoli, and roads are said to be blocked. There are also attempts to block certain roads in Beirut as well in support of Sheikh Assir."

Our correspondent said that there were also small protests happening across the country in support of the army, with some blocking roads in the Metn province, north of Beirut.

Taking a stand

The army said it would not stay silent on the attack, sent more troops to Abra, and called on politicians to take a stand.

"Political leaders across the country are rallying behind the Lebanese army, with the interior minister calling attacks on the army a 'red line'," said our correspondent.

In Tripoli and the Bekaa Valley, roads were cut off and gunmen roamed the streets.

In the Palestinian refugee camp of Ain al-Hilweh, at the edge of Sidon, radical groups exchanged gunfire with the Lebanese army.

Last week, the Sunni sheikh called on supporters to fire on apartments in Abra that he claimed housed Hezbollah members.

Abra is home to a mosque where Assir leads main Friday prayers. The sheikh believes Hezbollah uses the Abra apartments to keep him under surveillance.

His supporters clashed with Hezbollah in Abra last week that left one man dead.

Assir rose to prominence about two years ago over his opposition to Hezbollah and its ally, the Syrian regime of President Bashar al-Assad.

Syria-related tensions have soared in Lebanon, deepening sectarian rifts between Sunnis and Shias.