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Syrian regime forces repelled a huge Islamic State group counter-offensive in the desert region between Deir az-Zour and Homs provinces on Friday, but at a bloody cost for Damascus.

Around 120 regime fighters were killed when IS launched a surprise attack around al-Sukhnah killing around 120 Hizballah and pro-Damascus fighters in 24 hours of fierce fighting, according to the Syrian Observatory for Human Rights.

IS seized vital oil and gas fields, along with several towns and villages along the Euphrates River threatening to turn back recent advances from forces loyal to Bashar al-Assad.

Heavy Russian and Syrian airstrikes and shelling managed to halt the advance, while pro-regime fighters re-captured the vital supply route between Deir az-Zour and Palmyra on Friday.

"The Syrian army and its allies completely secured the Deir al-Zour-Palmyra highway after foiling the intense attack," Hizballah told Reuters. "The highway has become passable for traffic in both directions to and from Deir al-Zour."

Syrian state media said the regime had repelled several "ferocious" IS counter-attacks to secure the highway, but gave no details on the number of people killed in the battles.

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It comes after an audio recording was released, purported to be of IS leader Abu Bakr al-Baghdadi.

The defiant speech called on fighters to "stand fast" following recent advances from pro-regime and US-backed forces in Syria.

Pro-regime media said IS sent in huge numbers of militants from Iraq, where the group's territories have shrunk to a few pockets of land due to recent bloody Iraqi victories, including in the country's second city Mosul.

IS are also under attack from the US-backed Syrian Democratic Forces, where it has lost most of the city of Raqqa, the group's self-declared "capital".

Recent regime advances in Deir az-Zour has also seen it lost territories, with pro-Damascus forces breaking the sieges on the provincial capital and military airport, along with seizing gas and oil installations.