BEIRUT (Reuters) - Islamic State insurgents have opened an office in northern Syria where single women and widows can register to marry fighters from the radical al Qaeda offshoot, a monitoring group said on Monday.

The office in al-Bab, a town northeast of Aleppo city, records the names and addresses of the women so that Islamic State militants can go to their families and make a formal offer of marriage, the Syrian Observatory for Human Rights said.

The Observatory, which cited sources in al-Bab and Aleppo, is based in Britain and relies on a network of people on the ground in Syria to report information. It was not immediately possible to confirm the report independently.

There have been previous reports of militants seeking wives or forcing women to marry them in areas where the group has been active. The Observatory's director, Rami Abdurrahman, said it was the first time he had heard of the group setting up a formal office to register potential spouses.

Islamic State has set strict limits on women's rights in areas it controls. A group statement issued last week said women in the Iraqi city of Mosul, which the rebels captured in June, had been warned to wear full-face veils or risk severe punishment. The statement also listed guidelines on how veils and clothes should be worn, part of Islamic State's campaign to forcibly impose their radical brand of Islam.

The Sunni insurgents have declared a caliphate in parts of Iraq and Syria and have threatened to march on Baghdad.

Islamic State captured large tracts of land in neighboring Iraq last month and has advanced in parts of Syria where it has become one of the most potent armed groups in the country, three years into its civil war.

More moderate rebels launched a major offensive against Islamic State in January and have pushed them out of large swathes of Aleppo province, although the two sides still clash regularly.

Islamic State, formerly known as the Islamic State in Iraq and the Levant (ISIL), has mainly battled rival rebel groups in Syria but has also been confronting forces loyal to President Bashar al-Assad more frequently in the past six weeks.

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The United Nations says more and more Syrian rebels are joining the ultra-hardline group and estimates that there are 10,000-15,000 foreign fighters with various factions battling government troops.

On Sunday the Syrian army said it recaptured a gas field east of the central city of Homs that was seized by Islamic State insurgents earlier this month. [ID:nL6N0Q20Q2]

(Reporting by Sylvia Westall; Editing by Mark Heinrich)