Syrian government soldiers patrol a war-damaged area of Aleppo's historic center on Sept. 16, 2016. (Youssef Karwashan/AFP/Getty Images)

Clashes erupted Friday between Syrian rebels and government forces just 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 seriously threaten the truce or immediately spark wider unrest, although the fighting 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, however, 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 al-] Assad not giving the green light to trucks coming across the border” to reach Aleppo, an Obama administration official said in Washington.

The official said that 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 the condition of anonymity to discuss internal deliberations. “Until this aid comes through, I think it’s hard to see” implementation of other parts of the agreement.

A second phase of the deal spells out potential U.S.-Russia military coordination against militant groups, including the Islamic State and a former al-Qaeda faction. Implementation of that phase requires seven consecutive days of reduced violence in Syria as well as delivery of the aid.

In a morning telephone call with Russian Foreign Minister Sergei Lavrov, Secretary of State John F. Kerry warned that the United States would not establish the Joint Implementation Center, where the coordination is supposed to take place, “unless and until the agreed terms for humanitarian access are met,” said State Department spokesman John Kirby.

Washington expects Moscow to use its influence with Assad “to allow U.N. humanitarian convoys to reach Aleppo and other areas in need,” Kirby said in a statement.

Friday’s 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.

Rebels, however, claimed that Syrian troops attacked the area first, launching a three-pronged assault that began in the early morning. The Syrian Observatory for Human Rights, a Britain-based activist network, said the government targeted Jobar with surface-to-surface missiles.

“The regime forces tried to advance under the cover of tanks and mortars,” said Muhammad Abu Yaman, an activist with the opposition-aligned Jobar Media Center. “We never believed in the truce because we never trusted the regime.”

It was not possible to independently verify the accounts, although the Obama administration official charged that Assad was “the lead spoiler.” But the clashes highlighted the challenges facing the cease-fire’s broader objectives.

The United States, which has supported opposition rebel groups, and Russia, which backs the Assad government, have hoped the accord will pave the way for political negotiations to end the bloodshed.

While the truce has reduced the level of violence, disputes have emerged over the most basic tenets of the deal. Under the agreement, combatants are expected to withdraw from Castello Road, the only way in and out of the besieged part of eastern Aleppo. Each side has accused the other of failing to pull back.

A retreat from the road would allow U.N. agencies and other aid groups to bring lifesaving assistance to desperate civilians in Aleppo, now blockaded for more than a month. But a test of security arrangements along the road is possible only once the Syrian government allows the trucks to enter the country, the administration official said.

On Friday, a coalition of at least 100 Syrian and international nongovernmental organizations released a statement urging the United States and Russia to pressure the government and Syrian opposition to allow aid to get through during the rare calm.

“Sporadic and temporary cessations of violence cannot become ends in themselves,” said the statement, which was signed by organizations including Oxfam and Save the Children. “The lives of innocent Syrian civilians are in their hands.”

Brita Haji Hasan, president of the opposition council in Aleppo, said in an interview that residents also want access to Castello Road. There also are patients who need to be evacuated for urgent medical care.

The U.N. view is that “fuel and commercial traffic are, of course, important, but that it’s not under their jurisdiction,” Hasan said. He vowed that the opposition would not reject the U.N. aid.

“But what is happening is unreasonable,” he said. “There is no clear plan for anything.”

Heba Habib in Berlin and Karen DeYoung in Washington contributed to this report.