The death toll among Syrian government troops in airstrikes by the US-led coalition the day before has risen to 90, a monitoring group reported on Sunday.

The Syrian Observatory for Human Rights added that the raid, which the US said was unintentional, lasted 40 minutes, hitting several Syrian positions near an airbase in the eastern province of Deir al-Zour.

The Observatory initially estimated that 83 Syrian soldiers were killed and 120 injured in the Saturday attack that sparked a row between Russia, a key ally of Syrian President Bashar al-Assad, and the US.

US Defence Department spokesman Peter Cook said the pilots believed they were striking Islamic State targets, and that Russia had been notified earlier in the day at its plans to operate in the area but that "no concerns were voice."

Australia said Sunday that its planes were among the coalition aircraft over Deir al-Zour. "While Syria remains a dynamic and complex operating environment, Australia would never target a known Syrian military unit," the Defence Department in Canberra said.

Damascus said that the attack allowed the Islamic State terrorist militia to advance in the area.

Shortly later, al-Assad's forces, backed by Russian air power, recaptured the positions they had lost out to militants, the Observatory said.

Growing tensions between the US and Russia over the coalition bombardment of the Syrian military have cast doubts over a ceasefire that came into effect in Syria last week.

The truce was brokered by Washington and Moscow earlier this month as part of an ambitious plan aimed at halting fighting in Syria and relaunching a UN-sponsored peace process.

The truce excludes operations against Islamic State and al-Qaeda-linked militants.

In February, a ceasefire, brokered by the US and Russia, went into effect in Syria, but did not hold for long.

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