Syrian troops backed by warplanes battled rebels for control of strategic hilltop villages near the Lebanese border Friday, as government troops stepped up counterattacks against opposition forces threatening regime supply lines on the country’s frontiers.

Bomb blasts and shots fired into the air to mourn a fallen Syrian government soldier could be heard on the Lebanese side of the border as fighting raged around Qusair, a contested town near a key highway between Damascus and the coast.

The battles there came as government forces launched a second offensive against rebel forces in the southern province of Daraa, on the Jordanian border, where the opposition has been making steady advances in recent weeks.

While President Bashar al-Assad’s forces are stretched thin and much of the country has slipped into the rebels’ hands, the government is fighting hard to keep control of airports, seaports and roads linking both to the capital, Damascus.

Also Friday, activists said rebels clashed with troops in the northeastern border city of Qamishli, less than two miles from the border with Turkey. Fighting is rare in the predominantly Kurdish and Christian city, where rebels usually maintain a truce with the government.

It was not clear what prompted the clashes, which according to the Britain-based Syrian Observatory for Human Rights included members of the Islamic extremist Jabhat al-Nusra group.

The momentum achieved by the rebels in the past weeks is largely due to an influx of arms, according to Arab officials and Western military experts who say Mideast powers opposed to Assad have stepped up weapons supplies to Syrian rebels, with Jordan opening up as a new route.

While much of the recent fighting has been in Daraa, rebels have also made advances in Homs province near Lebanon. The province saw some of the heaviest fighting during the first year of the Syrian conflict, which erupted in March 2011, and intermittent episodes of violence since.

On Friday, sporadic explosions inside Syria echoed across the Lebanese side of the border and an Associated Press reporter said Syrian warplanes carried out at least one airstrike inside Syrian territory near Qusair.

Six people in the area, including two children, were killed when a shell struck their home, the Observatory said. Four rebel fighters were also killed.

The Washington Post’s Liz Sly and David Ignatius look back at the bloody Syrian civil war--thousands killed, a country in ruins and borders breached by a tide of refugees. What will the future hold for the Syrian people and the al-Assad regime, and how does the U.S. fit into that picture? (Jason Aldag/The Washington Post)

— Associated Press