The US-led coalition admitted it may have hit a Syrian army position Saturday as Russia called an emergency meeting of the UN Security Council to discuss the air strikes it said killed at least 62 soldiers.

The strikes came with diplomatic tensions escalating between Moscow and Washington less than a week into a fragile ceasefire aimed at stopping the bloodshed in Syria's five-year civil war.

American officials said the US-led coalition fighting the Islamic State (IS) group may have hit Syrian military positions.

"Coalition forces believed they were striking a Daesh fighting position," a Pentagon statement said, using an Arabic acronym for the IS group.

"The coalition air strike was halted immediately when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military."

Washington said it was investigating the incident but dismissed Moscow's call for an urgent Security Council meeting as a "stunt".

"If we determine that we did indeed strike Syrian military personnel, that was not our intention. And we of course regret the loss of life," US ambassador to the UN Samantha Power told journalists as the closed-door meeting got underway Saturday night.

Power described Russia's request for the meeting as a "stunt, replete with moralism and grandstanding," saying Moscow should instead demand a meeting with the Syrian regime to press for peace.

Russian ambassador Vitaly Churkin returned by accusing the United States of violating agreements that it would not target Syrian positions.

- 'Direct consequence' -

The Russian military earlier said the situation was deteriorating, adding that the United States would be responsible if the fragile ceasefire in force since Monday collapses.

The Syrian Observatory for Human Rights monitoring group gave a toll of 83 soldiers killed, as it confirmed the strikes were US-led coalition raids.

"Warplanes from the international anti-jihadist coalition carried out four air strikes today against Syrian forces surrounded by IS in the Deir Ezzor air base," a Russian army statement said.

"Sixty-two Syrian soldiers were killed and a hundred others were injured in these strikes."

The Russian military said two F-16 and two A-10 jets that flew into Syrian airspace from neighbouring Iraq carried out the 1400 GMT attack.

"Straight after the coalition's strikes, IS militants launched an offensive," said the statement, adding that "fierce fighting against the terrorists" ensued nearby.

"If these strikes were due to an error in the target coordinates, that would be a direct consequence of the US' refusal to coordinate with Russia its fight against the terrorist groups in Syria," it said.

The IS-linked Amaq news agency said coalition strikes hit IS positions, but that the jihadist group was able to "seize full control of Jabal Therdeh, which overlooks the Deir Ezzor airport".

The Observatory also reported that "20 members of IS were killed and dozens more wounded in heavy Russian strikes on Jabal Therdeh".

Syria's army has been fighting off a fierce IS offensive on the Deir Ezzor military airbase since last year.

A Syrian military source inside the Deir Ezzor airport told AFP US-led coalition strikes hit two hilltops near the airport.

- 'Situation worsening' -

Russian military officials meanwhile lashed out at both the United States and mainstream rebels in the strongest language yet over the ceasefire struck last week in Geneva, a last-ditch effort to stop the bloodshed in Syria.

"The situation in Syria is worsening," Russian General Vladimir Savchenko said in a televised briefing earlier.

"In the past 24 hours, the number of attacks have risen sharply," with 55 attacks on government positions and civilians, killing 12 civilians, he said.

In a statement, the Russian defence ministry accused what it termed "moderate rebels" of causing the ceasefire to fail.

Under the US-Russia deal, if the truce lasts seven days and humanitarian access is granted, Moscow and Washington are to work together to target jihadists including IS.

"If the American side does not take the necessary measures to carry out its obligations... a breakdown of the ceasefire will be on the United States," army general Viktor Poznikhir said.

Russian President Vladimir Putin, a key ally of Syrian President Bashar al-Assad, said earlier Saturday he remained "positive" about the truce but accused rebels of "attempts to regroup".

Putin said Washington apparently "has the desire to keep the capabilities to fight the lawful government of President Assad", calling it a "very dangerous path".

Implementing the truce has been complicated by the presence of jihadists not covered by the ceasefire and mainstream rebels on some of the same frontlines.

A challenge for Washington is to persuade opposition groups it backs to separate themselves from the former Al-Qaeda affiliate Fateh al-Sham Front, previously called Al-Nusra Front.

A key plank of the truce deal was the delivery of aid to areas including Aleppo, where an estimated 250,000 people in rebel-held areas of the city are living under government siege.

But 40 trucks carrying desperately needed food aid were still stuck on the border with Turkey on Saturday.

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