BEIRUT – Refugees poured into Arsal, a Lebanese town in the Bekaa Valley near Syria, on Saturday amid reports that a battle was beginning along the mountainous border, a sign that the government was broadening its offensive against rebel strongholds.

Heavy clashes were reported along the main highway from Damascus to Homs between the towns of Nabak and Qara, a corridor that is vital to both sides. It serves as a conduit for rebel fighters and arms from Lebanon and links Damascus to solidly government-held territory to the north along the Syrian coast.

Fighting in the area, known as Qalamoun, has long been expected, since the government took Qusair, another border town, in May. There were indications that government forces and their allies from the Lebanese militia Hezbollah were massing in the area, as were rebels from the Nusra Front and other groups, according to the Syrian Observatory for Human Rights, an anti-government monitoring organization.

The fighting threatens an area that until now had enjoyed a measure of autonomy under local rebels and opposition activists. In Yabroud, people of various sects and political views were living together, residents say. The area was also a haven for refugees from Qusair.

Lebanon, a country of 4 million people, is already struggling to absorb about 1 million Syrian refugees. Tensions have run high in the Bekaa Valley for months.

New York Times