Escalating violence in Syria is hampering UN efforts to expand aid operations and help more than 1 million people caught up in 15 months of turmoil, a UN official said.

Robert Watkins, UN Development Programme representative in neighbouring Lebanon, said the United Nations was still delivering assistance but plans to set up field offices in four of the worst-hit areas in the country were facing obstacles.

"Increasing violence has made it extremely difficult to establish the field presence so important for ensuring the delivery of humanitarian aid," Watkins told Reuters.

"Humanitarian deliveries do continue, however, though not at the pace we had hoped and as required by the needs."

UN aid agencies had been largely shut out of Syria during the uprising against President Bashar al-Assad which erupted with popular protests in March last year and has evolved into an armed insurgency.

More than 10,000 people have been killed, at least 500,000 have been internally displaced and the United Nations says at least 1 million are in need of help.

Two weeks ago Syria agreed to allow the United Nations and international agencies to expand their humanitarian operations in the country to address the growing needs.

Under the agreement, the United Nations plans to open field offices in four provinces which have been worst affected by the violence - Deraa, Deir al-Zor, Homs and Idlib.

But fighting has raged in all four areas, despite an April ceasefire deal brokered by international mediator Kofi Annan. UN observers who were sent to Syria to monitor the ceasefire were forced to suspend their work on Saturday because of the bloodshed and attacks on their own convoys.

On Thursday aid workers hoping to evacuate trapped civilians and wounded from the city of Homs were unable to enter because of continued shooting and shelling - in spite of another truce agreed by both government and rebel forces.

The team from the International Committee of the Red Cross and the Syrian Red Crescent eventually returned to Damascus after failing to get into the old city of Homs.