BEIRUT — Heavy fighting broke out in the Syrian capital between government forces and an insurgent group Friday in some of the most serious clashes since a U.S.-Russia brokered cease-fire went into effect four days earlier and brought relative calm to the war-ravaged country.

The fighting came as an opposition monitoring group reported that Russian troops have deployed along a main road leading into besieged rebel-held neighborhoods of the northern city of Aleppo, signaling the possible arrival of aid convoys later Friday.

The fighting and shelling in neighborhoods on the edges of Damascus were the heaviest in weeks, according to activists and residents, sparking concern the fragile cease-fire may be starting to fray.

Insurgents shelled government-held areas in the eastern Damascus neighborhood of Qaboun, wounding three people, Syrian state media said. SANA said the shelling violates the cease-fire.

The Britain-based Syrian Observatory for Human Rights said the fighting between government troops and rebels is concentrated in the neighborhood of Jobar, next to Qaboun where rebels have had a presence for years.

Mazen al-Shami, an opposition activist near Damascus, said government forces tried to storm Jobar but were repelled by opposition fighters. Al-Qaida and Islamic State group militants, who are excluded from the cease-fire, are not present in the area, he said.

“This is one of the most serious violations of the cease-fire,” al-Shami said via Skype.

SANA accused the insurgents of launching the attack, triggering retaliation by government forces.

The truce has been holding despite some violations, with the Syrian opposition on Thursday reporting 46 cease-fire violations around the country. The Observatory on Thursday reported the first three deaths since the cease-fire went into effect.