MOSCOW/BEIRUT (Reuters) - 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 dispute 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.

In another blow to hopes for the ceasefire, the governor of Homs said several hundred rebels would be evacuated from the last rebel-held district of the Syrian city on Monday, prompting rebels to warn any such step would amount to the government declaring the end of the truce.

On Saturday, the Russian Defence 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.

Syrian 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 President Bashar al Assad’s government.

The Britain-based monitoring group Syrian Observatory for Human Rights said a military source at Deir al-Zor airport reported at least 90 Syrian soldiers were killed in the strikes.

A U.S. official said the U.S. military believed reports that about 60 Syrian troops were killed but declined to speculate on the number of wounded. Two U.S. officials also confirmed that a tank was among the vehicles hit in the strike, raising more questions about what kind of intelligence led the coalition aircraft to conclude Islamic State militants were operating the military equipment.

“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,” Russia’s Foreign Ministry said in a 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.”

U.S. Secretary of State John Kerry in turn told CNN that Russia should do more to support the ceasefire and “stop the grandstanding, stop the showboating and get the humanitarian assistance going.”

Kerry called Saturday’s incident a “terrible thing ... that we all acknowledge and regret.”

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ACCUSATIONS AGAINST WASHINGTON

Russia, which support’s Assad along with Iran, has called on Washington to press the moderate Syrian opposition to separate itself 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, Syrian U.N. Ambassador Bashar Ja’afari said the U.S.-led coalition strikes were intended to sink the ceasefire.

French Foreign Minister Jean-Marc Ayrault said that despite the strikes on the Syrians, Syrian government forces were principally behind the truce violations.

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 forces 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 U.S. collusion with Islamic State.

The dispute 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

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, 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 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 humanitarian situation in besieged areas they control, and blamed Russia for undermining the ceasefire.

“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 militias a chance to regroup and deploy forces.

ISLAMIC STATE

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

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 toward 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, the same area as the 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.