BEIRUT - 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 Assad, but deliveries of aid have also been hampered by rebel forces who are surrounding two northern towns.

The UN Relief and Works Agency (UNRWA) said Damascus had authorized 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.

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