TRIPOLI - Overnight clashes killed six people in the Lebanese port of Tripoli, a security source said Thursday, as a fifth day of violence sparked by the Syria conflict spread to previously quiet neighbourhoods.

"Very violent fighting took place last night until 5:00 am (0200 GMT) that killed six people and wounded 40. The clashes and shelling affected several areas of the city, including the centre," the source told AFP.

Violence has regularly broken out in Tripoli since the beginning of Syria's uprising, pitting residents of the Sunni Bab el-Tebbaneh district against those from the neighbouring Alawite area of Jabal Mohsen. But since Sunday, shelling and clashes have spread to other mostly Sunni areas of the city, killing 17 people including two soldiers and wounding 150 others. An AFP correspondent said large parts of the city of 500,000 inhabitants were shut down on Wednesday, with schools and shops closed after the clashes.

Troops have been deployed across the city since the outbreak, but this has failed to halt the fighting.

The latest violence began as the regime of Syrian President Bashar al-Assad launched a withering assault on the rebel stronghold of Qusayr, near the border with Lebanon.

Lebanese Shiite militant group Hezbollah has been sending fighters across the border to help Syrian regime forces attack Qusayr.

And on Thursday, a Lebanese source close to Hezbollah told AFP that 75 the group's combatants have been killed in the Syrian conflict, especially in Qusayr.

Amin al-Qabbut, mukhtar (municipal official) of the Sunni Al-Qobba area, said areas of Tripoli last attacked during the Syrian army's bombardment of the northern city in 1985 were being shelled again.

"This war is the continuation of the 1985 war that Syria waged against us," Qabbut said.

In 1985, the Syrian army clashed with Sunni groups in Tripoli, and bombarded areas of the city, during Lebanon's civil war.

"The political tool used to wage the war is the same, it is the Arab Democratic Party," Qabbut said, referring to the party linked to Tripoli's Alawite community.

The ADP has, in return, accused Sunni groups of starting the fighting.

The Sunni-majority port has been the scene of intense clashes between Sunni supporters of the anti-Syrian opposition and Alawite Muslims loyal to a Hezbollah-led alliance backed by Iran and Syria.