KABUL, Afghanistan - The relief agency Medecins Sans Frontieres said today it is pulling out of Afghanistan, discouraged about a fruitless investigation into the slayings of five of its workers and fearful of new attacks.

The Nobel prize-winning group's decision to withdraw was the most dramatic example yet of how deteriorating security has crippled the delivery of badly needed aid and reconstruction in Afghanistan since the Taliban regime was ousted more than two years ago. MSF had already suspended most of its work after the June killings and recalled all foreign staff to Kabul, the capital.

"Today's context is rendering independent humanitarian aid for the Afghan people all but impossible," the international group said in a statement.

Medecins Sans Frontiers, or Doctors Without Borders, said it would explain its decision in detail during a briefing later today, but cited three main reasons: the dangers on the ground, disappointment that the investigation into the June killings has gone nowhere and what it called the U.S. military's use of humanitarian aid "for political and military motives."

U.S. and NATO troops run several so-called Provincial Reconstruction Teams across the country, under which soldiers provide health care, dig wells and perform other work normally carried out by civilians. Aid groups have long feared that the practice blurs the lines between relief work and soldiers' efforts to persuade local communities to provide intelligence on militants' movements.

In the June 2 killings of its workers, two men on a motorcycle stopped a clearly marked MSF vehicle on a rural road as they returned to the provincial capital from a clinic they were helping to run. The three Europeans and two Afghans inside were shot dead.

A purported Taliban spokesperson claimed responsibility, and accused the victims of working for American interests- a shock to MSF, which like many agencies relies on neutrality to protect staff.