An image made from video shows smoke rising from fighting near a main highway in Damascus on Friday. (Ugarit News/AP)

Rebels pushed forward in their battle with the Syrian army in Damascus on Friday, clashing with regime soldiers in contested neighborhoods and shutting down a key highway out of the capital with a row of burning tires, activists said.

A number of rebel brigades launched a series of attacks beginning Wednesday against regime checkpoints along the main road to from Damascus to northern Syria and have been clashing in the area since. The government has responded by shelling a number of rebel-held districts nearby.

A spokesman for one of the opposition groups fighting in the area said the rebels sought to open a path for a future assault on the city.

"This is not the battle for Damascus. This battle is to prepare for the entry into Damascus," he said via Skype, giving only his nickname of Abu al-Fida for fear of reprisals.

The fighting this week in Damascus, some of the heaviest since July, has brought the civil war that has destroyed entire neighborhoods of other Syrian cities closer to the heart of the capital, which has mostly been spared heavy fighting.

Both the rebels and the regime of President Bashar Assad consider the fight for Damascus the most likely endgame in a civil war that has already killed more than 60,000. The government controls movement in and out of the heavily defended city with a network of checkpoints, and rebels have failed so far to make significant inroads.

Abu al-Fida said one checkpoint on the highway changed hands twice on Thursday but was securely in rebel hands Friday. He said rebels were within a half-kilometer from Abbasid square and were firing mortars at a military base near the landmark plaza.

Online videos showed a row of burning tires laid across the highway, blocking all traffic. Smoke rose from a number of areas nearby, reflecting clashes and government shelling.

The videos appeared genuine and corresponded to activist reports.

Bus stop explosion kills 54

The Britain-based Syrian Observatory for Human Rights reported Friday that 54 people, including 11 women, were killed in a bombing at a bus stop near a military factory in the central city of Hama earlier this week.

The area — north of the war-torn city of Homs — is government-controlled, which is why reports on the blast were slow to emerge.

Observatory director Rami Abdul-Rahman said an explosive-laden mini-bus blew up at a bus stop near the factory in Buraq, near the central city of Hama, while workers were waiting for rides home. The factory makes military supplies, but not weapons, he said.

"These people work for the Ministry of Defense, but they are all civilians," he said, adding that no one from the military was killed in the blast.

Syria's state news agency reported the explosion on Wednesday evening, saying "terrorists" detonated a car bomb near a factory. It did not say what the factory produced or specify the number of dead and wounded. The regime refers to rebels fighting to topple the Assad regime as terrorists.

No one has claimed responsibility for the blast, which resembled others in recent months that appeared to target buildings associated with Syria's military and security services.

Foreign fighters flock to Syria

Some of the bombings have been claimed by Jabhat al-Nusra, an al-Qaeda-linked group fighting alongside the rebels which the U.S. had designated a terrorist organization.

As the situation in Syria has worsened, foreign jihadists have flocked to Syria to join what they consider a holy war to replace Assad's regime with an Islamic state in Syria. Most of the foreign fighters are Arabs from Egypt, Tunisia, Libya and other countries.

Late Thursday, the chief of the Netherlands' top intelligence agency warned that dozens of Dutch citizens are fighting with Syria's rebels and could return home battle-hardened, traumatized and radicalized.

General Intelligence and Security Service chief Rob Bertholee told the Dutch show Nieuwsuur that hundreds of people from around Europe, including dozens from the Netherlands, have travelled to Syria to join rebels fighting Assad.

He said propaganda romanticizing the civil war is helping draw foreigners into Syria's maelstrom of violence.

Syria's crisis began with peaceful protests in March 2011 and evolved into a civil war as the opposition took up arms to fight a government crackdown on dissent. The U.N. said last month that more than 60,000 people have been killed in the conflict.