Colombo - The United Nations office in Sri Lanka Thursday accused Tamil rebels of holding back its local staff members and demanded that they be allowed to return to government-controlled areas.

The staffers were part of a UN convoy which traveled to the northern Wanni area, on January 16, delivering urgent food and emergency supplies to displaced populations, the UN said in a statement.

The statement made no mention about the number of staff held back.

"The UN calls on the LTTE (Tamil rebels) to meet their responsibilities and immediately permit all UN staff and dependents to freely move from this area. The LTTE's denial of safe passage is a clear abrogation of their obligations under international humanitarian law," the statement said.

The convoy was the eleventh to take supplies to people trapped in the midst of fighting in the Vanni. Since early October UN convoys have brought approximately 7,000 tons of vital food and relief supplies to displaced populations.

Heavy fighting between government troops and Tamil rebels continue in the north-eastern Mullaitivu district, the last stronghold of the rebels. (dpa)