Syrian troops captured a contested suburb of Damascus on Wednesday as the government of Bashar al-Assad forged ahead with a military offensive that has already taken four other opposition strongholds south of the capital, state media said.

For more than a year, much of the belt of neighbourhoods and towns just south of Damascus has been a rebel bastion and a key arms conduit for the opposition. But Assad forces – bolstered by fighters from Lebanon's Shia militant group Hezbollah and Shia militants from Iraq – have made significant headway in the area in recent weeks.

The advances could give the regime a stronger position in proposed peace talks that the US and Russia have been trying to convene since May.

On Wednesday the town of Hejeira became the latest rebel-held suburb to fall into government hands. The state news agency Sana said the army had seized control of the town but was still battling rebels on its outskirts.

The UK-based Syrian Observatory for Human Rights said Assad's forces were in control of most of Hejeira but there were still small pockets of resistance.

Lebanon's al-Mayadeen TV, which had a reporter embedded with government forces in the offensive, broadcast what it said were live images from the streets of Hejeira, showing shattered storefronts, sandbags piled at street corners and the gutted concrete hulks of apartment buildings.

The opposition's hold on Hejeira became untenable after the military captured the adjacent town of Sabina in recent days.

While Assad's forces have driven the rebels from several of their footholds around the capital, the opposition is still within striking distance of the centre of Damascus, and fires barrages of mortar rounds into the city daily.

On Wednesday, mortar shells slammed into the Bab Touma and Zablatani neighbourhoods of Damascus, killing at least two people and wounding 20 others. The Observatory put the death toll at three.

In addition to its advances on the periphery of Damascus, the government has made inroads in recent weeks outside the northern city of Aleppo, wresting back a military base near the city's international airport as well as two towns along the road south-east of the airfield.

The Observatory director Rami Abdurrahman said government helicopters were dropping barrel bombs on rebel positions in Tel Hasel, the sole town along the road still in opposition hands.

Aleppo has been a major battlefield in the Syrian conflict since last summer, when rebels launched an offensive on the city. More than a year later it is now carved up into rebel- and government-held areas, and fighting has left much of the city in ruins.

The rebels are clearly concerned about the government's latest push: a group of six prominent rebel brigades has called for all fighters in the city to come together to repel the military offensive, activists say.

The armed opposition in Aleppo and the surrounding countryside has been crippled by recent infighting, which has undermined the rebels in their efforts to oust President Assad.

The government offensive falls against the backdrop of diplomatic efforts to convene a peace conference in Geneva to find a political solution to the conflict.

The main western-backed opposition group, the Syrian National Coalition, said this week it was ready to attend the proposed peace talks but only if certain conditions were met. It wants the government to allow humanitarian aid to reach besieged areas and to release political prisoners, demands the Assad government is unlikely to meet.

The prospects for the conference are further muddied by a dispute over a potential transitional government. The opposition, which has little support inside Syria, wants any future transitional government to exclude Assad and his close allies, a demand the Syrian government has rejected.