DAMASCUS, Syria -- Syrian government forces reached the outskirts of a key rebel-held town on Sunday, part of a weekslong offensive into the country's last rebel stronghold, state media and opposition activists said.

Further north, a car bomb exploded in a town held by Turkey-backed opposition fighters killing at least seven people.

Over the past two days, Syrian troops captured at least six villages near the strategic town of Maaret al-Numan in the northwestern province of Idlib. That brought them closer to retaking a critical north-south highway that passes through the town. It's been held by the rebels since 2012.

Syrian President Bashar Assad's government has retaken control of most of the country from rebel fighters, largely because of blanket air support from Russia, which helped turn the tide in the nearly 9-year civil war.

Idlib province is dominated by al-Qaida-linked militants. It's also home to 3 million civilians, and the United Nations has warned of the growing risk of a humanitarian catastrophe along the Turkish border. The government offensive in Idlib province has led to the displacement of hundreds of thousands of people, many of them to areas close to the border with Turkey.

According to opposition activists and paramedics, Maaret al-Numan is now almost empty as a result of the intense bombardment in recent weeks. Hadi Abdullah, an opposition activist based in Idlib, said Syrian warplanes and helicopter gunships were pounding areas near Maaret al-Numan. Opposition activists said government forces are now less than a kilometre (mile) away from the town.

Syrian state TV said government forces captured the village of Ghadqa near Maaret al-Numan early Sunday. Among the six nearby villages captured over the past two days were Tel Manas and Maarshamarin, according to the Britain-based Syrian Observatory for Human Rights and state news agency SANA.

In August, Syrian troops captured another town that the highway passes through, Khan Sheikhoun. If Syrian troops capture Maaret al-Numan, their next target is likely to be Saraqeb, which would become the last major town on the M5 highway outside government control.

Opening the highway would reduce travel time between Damascus and Aleppo by two hours, since drivers now must take a longer desert road.

Also Sunday, a car bomb in the northern town of Azaz killed eight people and wounded over a dozen more, according to SANA, when it went off outside a restaurant. The Observatory gave the death toll at seven people, with more than 20 wounded.

No one claimed responsibility for Sunday's attack in Azaz. Turkey-backed opposition fighters control the town. There have been a series of bombings over the past few months that left scores of people dead.

Last week, a blast killed three Turkish soldiers in northern Syria.

Turkey invaded the border area in neighbouring Syria to drive away the Syrian Kurdish People's Protection Units, or YPG, because of its alleged links to Kurdish rebels fighting inside Turkey. However, the same fighters were the main U.S. allies on the ground in the war against the Islamic State group.