Moscow stepped up its war of words with Washington on Sunday, saying air strikes by a U.S.-led coalition on the Syrian army threatened the implementation of a U.S.-Russian ceasefire plan for Syria and bordered on connivance with Islamic State.

The diplomatic row heated up on the last day of a seven-day ceasefire marred by a surge of violence as warplanes hit the strategic northern city of Aleppo for the first time since the truce came into effect.

On Saturday, the Russian defense ministry said U.S. jets had killed more than 60 Syrian soldiers in the eastern Syrian city of Deir al-Zor in four air strikes by two F-16 and two A-10 fighter jets coming from the direction of Iraq.

Syria’s U.N. Ambassador Bashar Ja’afari charged that U.S.-led strikes were aimed at torpedoing the ceasefire, but France’s foreign minister, speaking in New York, placed the main blame for truce violations on the government of President Bashar al Assad.

The Syrian Observatory for Human Rights, a Britain-based monitoring group with contacts across Syria, cited a military source at Deir al-Zor airport as saying at least 90 Syrian soldiers had been killed in the four air strikes.

Russia’s foreign ministry denounced the U.S. position on the incident as “unconstructive and inarticulate.”

“The actions of coalition pilots – if they, as we hope, were not taken on an order from Washington – are on the boundary between criminal negligence and connivance with Islamic State terrorists,” it said in a strongly-worded statement.

“We strongly urge Washington to exert the needed pressure on the illegal armed groups under its patronage to implement the ceasefire plan unconditionally. Otherwise the implementation of the entire package of the U.S.-Russian accords reached in Geneva on Sept. 9 may be jeopardized.”

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Russia, which along with Iran supports Syria’s Assad, has called on the United States to press units of the moderate Syrian opposition to separate themselves from Islamic State and other “terrorist groups.”

Iran also condemned the U.S. military action. “Such moves indicate America supports terrorist groups in Syria,” a foreign ministry spokesman said, according to Iranian news agencies.

In Venezuela where he was attending a summit, Syria’s U.N. Ambassador Bashar Ja’afari said the U.S.-led coalition strikes were intended to sink the ceasefire.

But French Foreign Minister Jean-Marc Ayrault said that despite the U.S.-led coalition air strikes on the Syrian army, it was Syrian government forces which were principally behind the truce violations.

“We have to hold on to this accord and keep it alive at all costs so we need to get over the events of the last few hours,” Ayrault told reporters at a ceremony at the site of the Sept. 11, 2001, attacks in New York.

“But while there were these incidents … we shouldn’t forget that what has harmed the American-Russian ceasefire is firstly the regime. It is always the regime of Bashar al-Assad.”

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The U.S. military said the coalition stopped the attacks against what it believed to be Islamic State positions in northeast Syria after Russia informed it that Syrian military personnel and vehicles may have been hit.

“The White House is defending Islamic State. Now there can be no doubts about that,” Russian foreign ministry spokeswoman Maria Zakharova said in comments aired by state TV.

The U.S. ambassador to the United Nations, Samantha Power, said Zakharova should be embarrassed by that claim. Russia’s U.N. representative Vitaly Churkin said Russia had no “specific evidence” of the U.S. colluding with Islamic State militants.

The diplomatic row should further complicate humanitarian aid deliveries to Syria, including its largest pre-war city Aleppo where the fragile truce is under threat.

Aleppo was hit by air strikes for the first time since the truce began. Moscow said militants there were preparing for large-scale military actions against Syria’s army.

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Heavy clashes continued on Sunday east of Damascus in the rebel-held Jobar suburb, the Observatory and a witness said.

The al-Rahman Legion, part of a Free Syrian Army rebel alliance there, said its fighters had destroyed a government tank and killed soldiers after government forces tried to storm Jobar for the second time this week.

The Observatory said at least eight people died and many were seriously injured when helicopters dropped barrel bombs onto a town in a rebel-held part of the southern Syrian province of Daraa on Sunday.

Insurgents say they only reluctantly accepted the initial deal to relieve the dire humanitarian situation in besieged areas they control, and blamed Russia for undermining the truce.

“The truce … will not hold out,” a senior rebel official in Aleppo said.

Rebels have also accused Russia of using the ceasefire to give the Syrian army and allied Shi’ite militias a chance to regroup and deploy forces ready for their own offensives.

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Islamic State is excluded from the truce. Separate U.S.-led, Damascus-led and Turkey-backed operations against the militants have continued throughout the ceasefire on various fronts.

One Turkish soldier and six Syrian rebels were wounded on Sunday in clashes with Islamic State near the Syrian border town of al-Rai as Turkey-backed Syrian rebels pushed south towards the IS-held town of al-Bab, Turkey’s Dogan News agency reported.

Turkey hit Islamic State targets within Syria with warplanes, according to Dogan, the Observatory and a rebel commander.

On Sunday, Islamic State said it had shot down a warplane in Deir al-Zor with “anti-aircraft” guns, in the same area as the U.S.-led coalition strikes hit the Syrian military on Saturday.

The Syrian military confirmed the loss of a warplane it said was carrying out an operation against rebels.