Children walk near damaged buildings in rebel-held Ain Tarma, eastern Damascus suburb of Ghouta, Syria. REUTERS/Bassam Khabieh (Bassam Khabieh/Reuters)

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 Monday in the launch of preparations between the United States and Russia for joint airstrikes against terrorist groups in Syria.

Instead, the violence ticked up Sunday, promised deliveries of aid failed to materialize and an errant strike Saturday by the U.S.-led coalition that killed dozens of Syrian government soldiers exposed the deficit of trust between the two powers.

Which countries were involved in the attack, which Russia said killed 62 Syrian soldiers, is unclear. The U.S.-led coalition comprises 67 countries, more than a dozen of which carry out airstrikes against the militants.

The Australian Defense Ministry, which is among those contributing to the effort, acknowledged in a statement Sunday that its warplanes had participated in a strike Saturday in Deir al-Zour, the eastern Syrian city where the attack occurred, on a front line between the Syrian army and the Islamic State that has changed hands many times.

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.

Russia sustained its verbal assaults on the United States on Sunday, with a 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.”

The statement stopped short of accusing the United States of deliberately ordering the attack to assist the Islamic State, but the main spokeswoman for the Russian Foreign Ministry had earlier made the allegation in televised comments, underscoring the depth of mistrust.

“We are reaching a really terrifying conclusion for the whole world: that the White House is defending Islamic State. Now there can be no doubts about that,” the spokeswoman, Maria Zakharova, said in comments broadcast by state television Saturday.

State Department spokesman John Kirby said the United States is pursuing the defeat of the Islamic State. “Any suggestion to the contrary flies in the face of the evidence,” he said.

The tensions erupted publicly Saturday evening at an emergency meeting of the U.N. Security Council, summoned by Russia, with the U.S. and Russian ambassadors walking out of each other’s speeches to the council to deliver scathing criticisms to reporters.

Yet although the strike was by far the biggest impediment to emerge since the shaky cease-fire went into effect a week ago, the truce was already not going well.

The terms had envisioned a ­seven-day period of calm during which the fighting would pause and humanitarian aid would flow unimpeded to stricken civilians living in areas under siege, and especially those living in rebel-controlled Aleppo.

As of Sunday, the sixth day of the cease-fire, trucks piled with food destined for rebel-controlled eastern Aleppo still had not been granted permission by the Syrian government to cross the Turkish border.

Meanwhile, warplanes presumed to be either Syrian or Russian returned to the skies over eastern Aleppo and dropped bombs on residential neighborhoods, killing at least one person, a child, and injuring several more people. Two children were among eight victims of a separate strike in a rebel-controlled area in the southern province of Daraa.

“We’ve seen reports of renewed airstrikes by the regime today in and around Aleppo,” Kirby said. “Obviously, if true, it is yet one more example of the Assad regime’s brutality and intent to use this period of reduced violence for territorial gain. We continue to urge Russia to use its influence on [Syrian President Bashar al-
Assad] to stop attacks on the moderate opposition and civilian targets, as well as to allow access to U.N. relief convoys to besieged areas.”

The Russian Defense Ministry accused the United States of failing to fulfill another key element of the plan — the delineation of territories controlled by moderate rebels and Islamist extremists, who in some places operate close to each other.

“Instead of the separation of the terrorists from the opposition promised by the United States, the militants are using the cease-fire regime to lick their wounds, restore the forces and prepare to resume the hostilities,” Maj. Gen. Igor Konashenkov, the Russian Defense Ministry spokesman, said in comments quoted by Russia’s Interfax news agency.

The faltering truce is expected to be a focus of discussions at the annual U.N. General Assembly meeting in New York, set to begin Monday. The truce was the result of months of intense negotiations between Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov, and the gathering may afford an opportunity for them to put the deal back on track.

DeYoung and Loveluck reported from Washington. Zakaria Zakaria in Istanbul and Carol Morello in New York contributed to this report.