BEIRUT —Trucks carrying humanitarian aid to Syria’s besieged rebel-held part of the city of Aleppo were held up for yet another day as heavy fighting broke out Friday on the edges of Damascus between government forces and insurgents.

The clashes were some of the most serious since a U.S.-Russia-brokered cease-fire went into effect this week. The fighting and mutual accusations of violations sparked concern that the fragile cease-fire, which brought some relief to millions of people across the war-ravaged country, may be starting to fray.

In a further sign of tensions picking up, the Obama administration warned Russia that potential military cooperation in Syria will not happen unless humanitarian aid begins to flow into Aleppo and other besieged communities.

The Russian military had said that the Syrian army withdrew its armor, artillery and other weapons from a key highway near Aleppo early Thursday, signaling the possible arrival of aid convoys after several days of delay.