Lebanese troops have shelled positions held by Sunni Syrian fighters in the mountains around the Lebanese town of Arsal on the Syrian border, in a third day of heavy fighting.

Monday's shelling came as Reuters reported the Lebanese army was advancing on Arsal, which was attacked by Syrian armed groups over the weekend.

The troops reportedly found 50 bodies of Syrian fighters, Reuters cited security sources as saying.

Sixteen Lebanese soldiers have died in the past three days along with dozens of fighters during the battles in Arsal.

At least six civilians have also died in the fighting.

Al Jazeera's Jony Tanios, reporting from Bekaa Valley, said the Lebanese army were sending reinforcements into the area.

"The government are sending more troops to Arsal, a huge number of troops entered in the past few hours to help the soldiers positioned around the town," he said.

"They are trying to put a stop to this situation as soon as possible, especially as there are a lot of civilians in Arsal."

The fighting came as hundreds of Lebanese civilians began fleeing the eastern border town.

It is the most serious spillover of violence from the civil war in Syria into Lebanon, the AP news agency reported.

Civilians were streaming out of Arsal in cars and pick-up lorries on Monday, fearing more violence as the Lebanese army brought reinforcements to the area.

About a dozen flatbed lorries carrying Lebanese army tanks were seen headed to Arsal's outskirts.

In addition to the soldiers killed in the clashes, about 13 Lebanese troops are missing after Syrian fighters made a cross-border raid and overran army positions in the area.

The Lebanese army has pledged to defeat the groups.

"What happened is far more dangerous than some believe," General Jean Kahwaji, Lebanon's military chief, said on Sunday.

"The terrorist attack which occurred yesterday was not an attack by chance or coincidence. It was planned previously, a long time ago, awaiting the appropriate time."

Tensions flare

Security officials say the attackers in Arsal include fighters linked to the Islamic State, an al-Qaeda offshoot that has seized territory in Syria and Iraq, as well as Syria's al-Qaeda branch, the Nusra Front.

Bombardment of the area prompted a large number of the tens of thousands of refugees sheltering in the area to flee, residents said.

Lebanon has been buffeted by the same waves of conflict that have brought about the rise of hardline Sunni groups in Syria and Iraq.

Kahwaji said Lebanon's army would resist any attempts to import Syria's war into his country.

Earlier on Sunday, fighting erupted briefly in Lebanon's main northern coastal city of Tripoli, where Syria's war has inflamed decades-old tensions between local Sunnis and Alawites.