ISTANBUL — Clashes erupted Friday between Syrian rebels and government forces outside Damascus, state media and activists said, puncturing days of calm under a cease-fire brokered by Russia and the United States.

The battles did not appear to threaten the truce or spark wider unrest, although the fighting between Syrian troops and anti-government rebels in the Damascus suburb of Jobar underscored the fragile hold of the cease-fire since it took effect Monday.

Of primary concern to U.S. and United Nations officials was the failure of aid agencies, for the fourth day in a row, to reach besieged populations in the city of Aleppo and other areas, including embattled sites near Damascus.

“The largest impediment is (Syrian President Bashar) Assad not giving the greenlight to trucks coming across the border” to reach Aleppo, an Obama administration official said.

The official said the Syrians were claiming that government offices were “closed” and unable to issue the required documentation because of the Muslim holiday Eid al-Adha.

Under the cease-fire deal, Russia is responsible for ensuring Syrian compliance with its terms.

“What we think is going on is that the Russians don't appear to have the leverage on Assad that they said they have,” said the official, who spoke on condition of anonymity.

“Until this aid comes through, I think it's hard to see” implementation of other parts of the agreement.

The violence near Damascus was the fiercest reported this week.

The state-run Syrian Arab News Agency said militants attempted to launch raids on military checkpoints near the capital.

The army responded with a counteroffensive, the agency said.