BEIRUT — Syrian troops clashed with rebels in a mountainous western region Saturday in what appeared to be an offensive to cut an opposition supply route from Lebanon, forcing hundreds to flee for safety across the border, activists and officials said.
The fighting was concentrated in the rugged Qalamoun region around the towns of Qara, Rima, and Nabak, activists and state media said. The battle has been expected for weeks as troops and opposition fighters reinforced their positions ahead of winter, when much of the area is covered with snow.
The Britain-based Syrian Observatory for Human Rights said the fighting had closed the highway linking Damascus with the central city of Homs. It said Syrian warplanes struck rebel held areas around Qara and nearby mountains.
The Observatory added that Lebanon’s militant Hezbollah group has deployed thousands of fighters on the Lebanese side of the border in preparation for the battle. Hezbollah’s fighters openly joined Syria’s civil war earlier this year, tipping the war in favor of government forces mostly in the central province of Homs and in the suburbs of Damascus.
Syria’s state-run news agency said troops had killed ‘‘tens of terrorists’’ in attacks on rebel hide-outs in areas including Nabak and Rima. The government refers to rebels fighting to overthrow President Bashar Assad as terrorists.
The Qalamoun region is a main rebel supply route for weapons and fighters from neighboring Lebanon. Government forces appear to be trying to regain control of the whole border with the country.
Hundreds of Syrian men, women, and children have crossed the border into the Lebanese town of Arsal seeking refuge from the fighting, the town’s former mayor Bassel Hojeiri said by telephone.
‘‘We have a major crisis,’’ Hojeiri said about his hometown, already thronged with thousands of refugees from the past two years of violence in Syria. ‘‘Most of those who fled came yesterday from Qara.’’
The Lebanese army said in a statement that troops captured nine Syrians as they tried to cross into Lebanon carrying weapons and grenades. It said an Algerian was also detained near the border and had no legal permit to stay in Lebanon.
The battle around Qara began Friday, said a rebel activist near the town who goes by the name Abu Yazan. Activists rarely give their real names, fearing identification by Syrian security forces or extremists.
If government troops gain the upper hand, they will be able to cut supplies that flow from Lebanon to rebel-held areas around Damascus, said the Observatory’s Rami Abdurrahman.
‘‘The support that comes from Lebanon is for the Qalamoun,’’ said Abu Yazan. He said the clashes were the most violent that had occurred in the area for months.
In eastern Syria, rebel groups seized one of the country’s main gas plants in Deir al-Zour, which supplies power stations feeding much of the country’s east, two activists said.
Abdurrahman and an Aleppo-based activist who is originally from the city said an alliance of Islamic rebel groups took the facility late Friday. The group is dominated by the Al Qaeda-linked Nusra Front, Abdurrahman said.
The Aleppo-based activist, who requested anonymity for fear of reprisals, sent the Associated Press a statement he said was from the alliance, confirming they had seized the plant. He said other activists in the city also confirmed the incident.
The facility was previously in the hands of tribal gunmen allied with Assad forces.