A day after re-entering the city, Isis has retaken Palmyra from government troops, the militant group and a Syrian government official have said.

After government troops and Russian air raids pushed the group out of the city, Isis militants regrouped and attacked from multiple fronts, forcing government troops to retreat, the Associated Press reported.

Homs Province Governor Talal Barazi told the pan-Arab Mayadeen news channel that the Isis attack on Palmyra is a "desperate" reaction to the Syrian government military "victories" on the ground and that the group wanted to "realise some type of gain".

Despite dozens of Syrian troops reportedly being killed in fighting around Palmyra, so far there is no hint the army would move resources away from Aleppo for the fighting in central Syria.

"I don't think the regime would withdraw forces from Aleppo to Palmyra and risk losing Aleppo," said Rami Abdurrahman, the head of the opposition monitoring group the Syrian Observatory for Human Rights.

"I think the regime's priority now is to finish the battle for Aleppo before the end of the month for sure. As for Palmyra, the whole international community would stand by it against IS," AP reported.

Mohammed Hassan al-Homsi, who runs Palmyra News Network, said Isis is refocusing efforts away from north Syria where the international coalition is and looking for new terrain. Palmyra has symbolic and strategic significance due to its desert terrain linked to Iraq and its surrounding oil and gas fields.

State news agency SANA reported that the militant group got reinforcements from Raqqa, enabling it to attack with "large numbers" against military checkpoints around the city.

Meanwhile, Russia's Defence Ministry partially blamed the US-led coalition for the advance into Palmyra, saying that the scaling down of its operation against Raqqa allowed thousands of Isis fighters to escape from Mosul.