Aid workers in Afghanistan are increasingly under threat, the United Nations has said, calling it a worrying trend as most US-led troops prepare to leave the country at the end of next year.

The departure of Western troops will leave Afghan forces to fight the Taliban-led fighters on its own, a security concern for foreign workers and their Afghan colleagues working on development and reconstruction projects across the nation.

"I am extremely concerned with this trend at a time when the country is in the midst of a difficult transition that may lead to increased humanitarian needs," UN Humanitarian Coordinator in Afghanistan, Mark Bowden, said in a statement on Saturday.

A total of 36 aid workers have been killed in Afghanistan so far this year, the UN said. It gave no comparative figures but said the trend highlighted the growing risk surrounding the delivery of aid.

Bowden's comments followed the execution of nine aid workers in two separate incidents in Afghanistan this month.

According to the Aid Worker Security Database, 73 humanitarian workers have been killed, kidnapped or injured in Afghanistan since the start of the year, more than recorded for the whole of 2012.

The figure is also the highest since the organisation began collecting the data in 1997.

In an attack on November 26, three aid workers were blown up by a remote controlled bomb in southern Afghanistan.

In the second attack on November 27, six aid workers with a French aid group in the west were shot by gunmen in an ambush.

In a deadliest year for aid workers since the Taliban were ousted in 2001, both civilian and security force casualties have also soared in Afghanistan this year.

Source: Reuters