* Convoy turns back after coming under fire
* Aid group says rebel infighting also blocking distribution
* U.N. says Foreign Ministry has promised more aid trips (Adds opposition aid group halts distribution, U.N. trip planned, donors pledge $2.4 billion)
By Oliver Holmes
BEIRUT, Jan 15 (Reuters) - Shooting forced the United Nations to abort a delivery of food and polio vaccines to a besieged area of Damascus after the Syrian government insisted it use a dangerous route, a U.N. spokesman said.
Aid workers in Syria have accused authorities of hampering deliveries to rebel-controlled areas and threatening groups with expulsion if they try to avoid bureaucratic obstacles to help people trapped in an almost three-year-old civil war. Syria blames rebel attacks for aid delays.
The U.N. Relief and Works Agency (UNRWA) said Damascus had authorised a six-truck convoy to deliver food for 6,000 people, 10,000 doses of polio vaccine and medical supplies to the Yarmouk Palestinian district where 15 people are known to have died of malnutrition and 18,000 are trapped by fighting.
UNRWA spokesman Chris Gunness said in a statement Syrian authorities had "required" that it use the southern entrance to Yarmouk.
That meant it had to drive 20 km (12 miles) "through an area of intense and frequent armed conflict, in which numerous armed opposition groups, including some of the most extreme jihadist groups, have a strong and active presence".
When the convoy passed the southern checkpoint, its Syrian government escort sent a bulldozer to clear the road of debris and it was fired on by unknown assailants.
Machinegun fire erupted and a mortar exploded close to the convoy, Gunness said, and UNRWA's Syrian security detail told the convoy to withdraw. No one was wounded, he said.
Yarmouk residents have been trapped for months with rebels under a government siege. Opposition activists say the government is using hunger as a weapon of war against its people and the United Nations has called for greater access.
According to the Syrian government, rebels are to blame for firing on aid convoys.
"Amid reports of widespread malnutrition in Yarmouk, amid reports of women dying during childbirth because of shortages of medical care, amid reports of children eating animal feed to survive, this is what happened to the UNRWA convoy," Gunness said.
"This is an extremely disappointing setback for the residents of Yarmouk who continue to live in inhumanely wretched conditions."
The U.N. spokesman in Damascus, Khaled al-Masri, said in an emailed statement later on Wednesday that U.N. agencies would send urgent food, medicine and winter relief to 20,000 people in areas to the east and west of Damascus on Thursday, districts also besieged by Assad's forces.
The statement said that the trip, in coordination with the Syrian Arab Red Crescent, had been agreed on with the Foreign Ministry and that further access had been promised.
It said the U.N. "hopes to be able to send similar assistance to other areas that have not been previously accessed, and this is what has been agreed (with the Foreign Ministry)."
International donors pledged more than $2.4 billion on Wednesday for the U.N. aid efforts in Syria, U.N. Secretary-General Ban Ki-moon said, following the largest appeal in U.N. history last month, for $6.5 billion.
At the conference in Kuwait, U.N. humanitarian chief Valerie Amos said all sides in the conflict had shown "total disregard for their responsibilities under international humanitarian and human rights law".
Aside from government restrictions, a member of an opposition aid group in northwest Syria said his organisation had cancelled shipments of flour and food in Idlib province due to internecine fighting among rebel groups.
Tarek Abdul Haq said the clashes between the al Qaeda-linked Islamic State of Iraq and the Levant and other groups had made deliveries too dangerous as aid workers could be accused by either side of working for the other.
"As an activist, I am not able to cross (rebel) checkpoints from either side for fear of arrest," he said from the Turkish border. (Additional reporting by Erika Solomon in Beirut and Sylvia Westall and Warren Stobel in Kuwait; Editing by Andrew Roche)