"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 center," the source told AFP.

Violence has regularly broken out between pro and anti-Syria government in Bab el-Tebbaneh and Jabal Mohsen neighborhoods of Tripoli since the beginning of unrest in neighboring Syria.

But since Sunday, shelling and clashes have spread to other areas of the city, killing 17 people including two soldiers and wounding 150 others.

Reports say 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 Syrian troops launched a withering operation in al-Qusayr town, near the border with Lebanon.