* Fifteen people have died in besieged suburb
* Convoy turns back after gun fight erupts
* U.N. says convoy did not get permission for safer route
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 said it should use a circuitous and dangerous route, a U.N. spokesman said.
Aid workers in Syria have accused authorities of hampering deliveries to opposition-controlled areas and threatening groups with expulsion if they try to avoid bureaucratic obstacles to help the tens of thousands trapped in an almost three-year civil war. Syria blames rebel attacks for aid delays.
Most are stuck due to sieges by forces loyal to President Bashar al-Assad, but deliveries of aid have also been hampered by rebel forces who are surrounding two northern towns.
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 have died of malnutrition and 18,000 are trapped by fighting.
UNRWA spokesman Chris Gunness said in a statement the Syrian authorities "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 one 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.
Syria is home to half a million Palestinians, refugees of the 1948 conflict which led to the creation of the state of Israel. Before the 2011 uprising against Assad, many lived in Yarmouk on the southern edge of the Syrian capital.
Since then, around 70,000 Yarmouk residents have fled.
Those that remain 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 have 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." (editing by Elizabeth Piper)