BEIRUT, Lebanon - Forces loyal to Syrian President Bashar al-Assad have firmly seized the momentum in the country's civil war in recent weeks, capturing one rebel stronghold after another and triumphantly planting the two-starred Syrian government flag amid shattered buildings and rubble-strewn streets.

Despite global outrage over the use of chemical weapons, Assad's government is successfully exploiting divisions among the opposition, dwindling foreign help for the rebel cause and significant local support, all linked to the same thing: discomfort with the Islamic extremists who have become a major part of the rebellion.

The battlefield gains would strengthen the government's hand in peace talks sought by the world community.

Both the Syrian government and the opposition have said they are ready to attend a proposed peace conference in Geneva that the United States and Russia are trying to convene, although it remains unclear whether the meeting will indeed take place. The Western-backed opposition in exile, which has little support among rebel fighters inside Syria and even less control over them, has set several conditions for its participation, chief among them that Assad must not be part of a transitional government - a notion Damascus has roundly rejected.

"President Bashar Assad will be heading any transitional stage in Syria, like it or not," said Omar Ossi, a member of Syria's parliament.

The regime's recent gains on the outskirts of the capital, Damascus, and in the north outside the country's largest city, Aleppo, have reinforced Assad's position.

On Saturday, Syrian troops clashed with rebels in a mountainous western region 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.

Earlier Saturday, activists said fighters from an al-Qaeda-linked Syrian rebel group beheaded an allied commander whom they mistook for a pro-regime fighter.

It was the latest excess attributed to the aggressive and radical Islamic State of Iraq and the Levant.