Rebels in Syria have staged a surprise attack against the key district of Baba Amr in the city of Homs, a year after regime forces retook it after a deadly month-long siege, activists have said.

The assault came as rebels in the oil-producing east of the war-ravaged country said they had established local religious committees to administer the area, including policing, the judiciary and emergency services.

"At dawn, the rebels launched a surprise attack on Baba Amr, which they have entered," Syrian Observatory for Human Rights director Rami Abdel Rahman, who was in contact with the rebels, told the AFP news agency on Sunday.

The assault on a key hub of the uprising against President Bashar al-Assad's government also comes two years after the outbreak of peaceful protests against his rule descended into armed conflict, when the regime responded with a
brutal crackdown.

Government forces pounded Baba Amr relentlessly for more than a month in 2012, seeking to oust rebels holed up in Homs.

Assad's troops eventually retook the district on March 1 last year after a bloody campaign.

The president himself last toured the battered neighbourhood on March 27, assuring residents who had stayed that it would be rebuilt and that normal life would resume.

Religious council

In the east of the country, where large swaths of territory are now under rebel control, groups including the al-Nusra Front have set up a religious council to administer affairs, the observatory said on Sunday.

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"God commanded the Islamic battalions to form a religious council in the east to administer the affairs of the people and fill a security gap," the groups said in a statement distributed by the Britain-based watchdog.

The council will include several offices charged with functions including justice, policing and emergency services, it said.

Video footage showed a convoy draped with black flags bearing Islamic inscriptions in the Deir Ezzor area and rebels attaching a banner to a building in Mayadeen, on which is written "Religious Committee of the Eastern Region."

Rebels in Deir Ezzor and other eastern provinces such as Hassaka and Raqa have made significant military gains as they battle Assad's forces.

Free Syrian Army fighters, composed mainly of army deserters, have told AFP that despite being fewer in number, the al-Nusra fighters, who were unknown before the rebellion, have better logistic and economic backing and receive financing "from abroad."

The group, which would like to see Syria become an Islamist state, has focused on strategic targets in the east, such as oil wells, and also recruits and pays local fighters.

Damascus accuses countries such as Saudi Arabia and Qatar of financing the rebel groups battling the regime.

It labels all armed opposition groups as "terrorists," in a conflict which the UN says has left more than 70,000 people dead since it erupted in March 2011.