BEIRUT (Reuters) - Thousands of new fighters joined Islamic State in Syria last month in its fastest expansion to date, a body monitoring the war said on Tuesday.

Now in control of roughly a third of Syria and large areas of Iraq, Islamic State has been seizing territory from rival Islamist groups in a belt of territory north of Aleppo, threatening rebel supply lines into the city where President Bashar al-Assad's forces are seeking to encircle the insurgents.

Islamic State recruited at least 6,300 men in July, Rami Abdelrahman, founder of the Syrian Observatory for Human Rights, told Reuters - a big expansion from early estimates suggesting the group numbered around 15,000. Around a thousand of the new fighters were foreign, and the rest Syrian, he said.

The surge followed Islamic State's rapid advance in northern Iraq in June, where its capture of the city of Mosul furnished it with new weaponry and resources, some of which were diverted to Syria. The group, now the target of U.S. air strikes in Iraq, has declared an Islamic caliphate in areas under its control.

Much of the recruitment has taken place in Islamic State's stronghold of Raqqa. The city on the Euphrates river in northeast Syria has been the target of dozens of air raids by government war planes in recent days.

Anecdotal accounts have also suggested a rapid increase in the group's numbers.

"I can tell it’s not far fetched that Islamic State gained thousands of new supporters recently with its operations in Deir al-Zor," one Raqqa resident, an activist opposed to both the Syrian government and the Islamic State, told Reuters by Skype.

Deir al-Zor province is southeast of Raqqa and another area of Islamic State control on the Euphrates river.

Islamic State last week staged an advance westwards in an area of territory it is seeking to control alongside the Turkish border. It seized several villages and towns north of Aleppo from rival Islamist groups in what observers believe is a push towards the border town of Azaz.

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The Observatory said Syrian war planes mounted air strikes on Islamic State positions in the town of Akhtarin on Tuesday, one of the areas its fighters seized in last week's advance. There were no reports of further air strikes on Raqqa.

Rebel brigades from insurgent-held territory in the northwest province of Idlib are now redeploying to meet Islamic State's advance towards Azaz, according to activists familiar with rebel movements.

(Writing by Tom Perry; Editing by Robin Pomeroy)