ISIL fighters have killed at least 34 Syrian government troops and seized their vehicles, pushing back the army advancing on Maadan, one of the last ISIL-held towns in Raqqa province.

The UK-based Syrian Observatory for Human Rights said the early Friday counterattack by the Islamic State of Iraq and the Levant (ISIL, known as ISIS) group also left 12 of its fighters dead.

The development set the government back about 30km to the west of Maadan and allowed ISIL to recapture a number of villages in the areas it lost to advancing Syrian soldiers last month.

ISIL media channels posted images of the attacked Syrian army convoy and bodies of more than a dozen soldiers.

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Maadan lies halfway between Raqqa city and Deir Az Zor city, where government forces control about half the city and a nearby airbase, but both of them are besieged by ISIL.

US-backed Syrian fighters are bearing down on Raqqa city from all sides. Half of the estimated 20,000 people trapped there are children, an official with the UN children's agency said Friday.

Fran Equiza spoke to The Associated Press in Damascus following a visit to three camps in northern Syria where he met displaced children from Raqqa and Deir Az Zor.

"I was completely overwhelmed," he said. "There are 10,000 children trapped in Raqqa in extremely dire conditions. No electricity, no water, probably very little food ... and the battle almost every day. The level of suffering, losing friends, relatives, family of these children is absolutely staggering."

On Friday, the Russian military, which provides air support to the Syrian army, said it was currently focusing on the government's offensive in Deir Az Zor.

"Breaking the blockade of the city will mark the defeat of the most capable part of the IS in Syria," said Sergei Rudskoi of the military's general staff.

Government gains

Speaking at a news conference in Moscow, he said the two-year Russian campaign has allowed Syrian President Bashar al-Assad's forces to quadruple the territory under their control.

The area under Syrian government control has increased from 19,000 to 78,000sq km since Russia launched its air raids in September 2015.

Russian pilots have flown more than 28,000 missions since the campaign's launch, he said.

Also on Friday, ISIL failed to push back against government advances in the central Syrian town of Akerbat.

Syrian troops have encircled ISIL fighters in Akerbat, according to Russian officials.

General Sergei Surovikin, head of Russia's military in Syria, said controlling Akerbat would allow the Syrian government to take full control of the oil and gas fields north of Palmyra.