Mansehra: Long-haired gunmen burst into the white stone building and killed four charity workers helping earthquake victims, then wrecked the office with grenades and set it on fire. Police came, but did not intervene.

In a tactic reminiscent of neighbouring Afghanistan, Islamist militants are targeting aid groups in the volatile northwest, and local authorities appear incapable - or unwilling - to stop them.

The threat has forced several foreign agencies to scale back assistance to survivors of the October 2005 earthquake that killed at least 78,000 and left 3 million homeless - risking the region's recovery from the worst natural disaster in the country's history.

The February 25 attack on employees of Plan International, a British-based charity that focuses on helping children, was the worst in a series of threats and assaults on aid workers in the northern mountains where Taliban-style militants have expanded their reach in the past year.

Menacing letters

Nearly a month later, menacing letters are still being sent to aid organisations. Although all four victims in Mansehra were Pakistani men, the extremists despise the aid groups because they employ women and work for women's rights.

Officials in Mansehra, speaking on condition of anonymity, said letters from extremists distributed on March 13 and 14 also warned schools to make sure girl students were covered from head to toe and to avoid coeducation.

The militants also may be trying to discredit the government and to enforce a radical religious agenda in a region where jihadist-linked groups were themselves a source of aid.

Police accuse local militant, Mohiuddin Shakir, who goes by the alias Mujahid, of masterminding the attack last month in Mansehra.

Shakir, a former member of an Al Qaida-linked group, now leads a jihadist group called Lashkar-e-Ababeel.

Brigadier Waqas Iqbal Raja, the chief security official for the Earthquake Reconstruction and Rehabilitation Agency, acknowledged a growing presence of extremists in the quake zone, including some militants displaced by an army offensive against supporters of a pro-Taliban cleric in neighbouring Swat district.

Tribal elders have promised to help protect aid workers, and Raja said he has advised the workers to keep a low profile and stay off the roads at night.