Dozens 'killed' in IS-Nusra Front battles near Lebanon border
The fighting is said to be approaching refugee camps in the rugged mountainous region, and is involving rocket and artillery barrages.
The Lebanese army seems to have also intervened, attacking both factions in the region, according to local witnesses.
IS has reportedly captured more positions previously held by Nusra and is advancing close to the refugee camps, prompting a large number of refugees to flee in the direction of Arsal.
Dozens of militants, mostly from the Nusra Front, have been killed in the ongoing fighting.
The source said the positions are close to the town and lead towards the area that intersects the Syrian border and the western Qalmoun front.
He added that rockets and heavy artillery were mainly used in the clashes.
The latest developments risk further embroiling the Lebanese army in the Syrian conflict, as they maybe dragged into directly confronting IS.
The National News Agency of Lebanon has reported that the army will target insurgents in the area, as other news outlets said additional military reinforcements were to be sent there.
In December a prisoner swap between the Lebanese army and al-Nusra took place in Arsal, from which the Lebanese soldiers were kidnapped the previous year.
The mountains neighbouring the restive border town have become intertwined with Syrian territories under the control of armed groups.
"Arsal on the eastern borders with Syria is an occupied area," said Lebanon's interior minister, Nohad Machnouk said last year, adding that the town "has 120,000 Syrian refugees, or more than the number of its population" living there.
"Thousands of armed militants are present in [its] barren mountainous areas," said Machnouk.