Updated 4.45pm

The governor of Syria's Homs province told state television today that the army was fighting to regain control of the ancient city of Palmyra after Islamic State had captured earlier in the day.

In the government's first official admission that Palmyra had fallen once again to the militants, Ikhbariyah TV quoted Governor Talal Barazi as saying the army had pulled out of the city.

"The army is using all means to prevent the terrorists from staying in Palmyra," he was quoted as saying, hours after IS and a Britain-based monitoring group both said the militants had full control of the city in his eastern province.

Earlier on Sunday, Islamic State militants and Syria's Russian-backed army both claimed they had the upper hand in the fight for the city.

Russia said its jets had helped force the militants out of the city centre overnight and its allies in the Syrian army were now fighting off another assault by the hardline Islamists.

But a news agency linked to Islamic State then said it had only briefly retreated and was now back in control of Palmyra, an account backed by the British-based Syrian Observatory for Human Rights, which monitors the conflict.

Palmyra, the site of a Roman-era city and spectacular ruins in the centre of Syria, has become an emblematic battleground in a civil war now in its sixth year.

Forces allied to Syria's government first recaptured the city from Islamic State in March, a victory held up as a major turning point in the war and the biggest reversal for the militants since Russia's intervention to support Damascus.

But Islamic State militants launched a surprise advance on the city on Thursday, taking control of nearby oil and gas fields and pushing towards an airbase used by Russian forces, the Observatory said.

Russia's defence ministry said its jets had launched 64 strikes and killed more than 300 militants overnight, helping the Syrian army push the main force back.

More than 4,000 Islamic State militants had since regrouped and launched a second attack on Sunday, Russian news agencies cited Moscow's monitoring centre in Syria as saying.

"Despite heavy losses in manpower and equipment, the terrorists are trying as hard as possible to secure a foothold inside the city," Interfax quoted a statement from the centre as saying. "Syrian troops are fighting to defend Palmyra."

Syria's army acknowledged there was a large offensive by the militants from several fronts near a major grain silo 10 km (6 miles) east of the city.

An Islamic State recapture of Palmyra would be a major reversal for Syria's government and its Russian backer, which hailed the city's capture in March, sent troops to protect it and even staged a concert there.