BEIRUT – A U.S.-Russian cease-fire deal for Syria was on the brink of collapse Sunday after a week of mishaps and setbacks that exposed the fragility of the plan.

The cease-fire is premised on a series of trust-building exercises that were intended to culminate today in the launch of preparations between the United States and Russia for joint airstrikes against terrorist groups in Syria.

Instead, an errant strike Saturday by the U.S.-led coalition against the Islamic State that mistakenly killed dozens of Syrian government soldiers has exposed the deficit of trust between the two powers.

Whether U.S. warplanes conducted the attack is in question. The U.S.-led coalition is made up of 67 countries, more than a dozen of which carry out airstrikes against the militants.

But the strike sent tensions soaring between Moscow and Washington, the chief sponsors of the truce, casting further into doubt the likelihood that they will be able to work together to end Syria’s war.

On Sunday, Russia continued its verbal assaults on the United States, with a Russian Foreign Ministry statement accusing the pilots who carried out the strikes of acting “on the boundary between criminal negligence and connivance with Islamic State terrorists.”

In Aleppo on Sunday, warplanes dropped barrel bombs on four residential neighborhoods, upending the relative calm that has prevailed there for a week. Aid convoys remained stalled on the Turkish border for a sixth consecutive day.

Russia and Syria asserted that 62 were killed and about 100 others were injured. Although the Central Command statement did not mention casualties, a senior administration official said the United States had “relayed our regret” through Russia “for the unintentional loss of life of Syrian forces fighting ISIL,” an acronym for the Islamic State, and had launched an internal investigation.

Deir al-Zour, where the airstrike took place, is far from the populated western region, where the separate civil war that is the subject of the cease-fire is focused. The Islamic State controls much of the province, although there are some scattered Syrian military installations.