KABUL, Afghanistan — United Nations officials said Monday that fighting arising from a Pakistani military crackdown in one of the semiautonomous tribal districts on Pakistan’s tense border with Afghanistan had driven 20,000 Pakistanis to flee as refugees into Afghanistan.

The exodus from the Bajaur tribal agency into Kunar Province in eastern Afghanistan echoed earlier waves of war-driven migration across the border, but in the opposite direction. After the Soviet invasion of Afghanistan in 1979, an estimated five million Afghans fled as refugees to neighboring countries, the largest number to Pakistan.

Officials of the United Nations High Commissioner for Refugees told reporters in Kabul that the United Nations and other aid agencies had rushed emergency supplies to the Kunar refugees, including food, plastic sheets, blankets and lanterns, and that medical assistance had also been provided.

The reverse flow of refugees into Afghanistan has its origins, at least in part, in the American-led invasion of Afghanistan in 2001. Many refugees who fled Afghanistan for Pakistan in the 1980s have now returned home. But many of the militants from the Taliban and Al Qaeda, who are fighting the NATO force that seeks to pacify Afghanistan, operate from sanctuaries in the Pakistani tribal areas.

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Many border tribesmen have never reconciled themselves to the Durand Line, the 19th-century, British-drawn border that divided the mainly Pashtun tribes of the area between British India — later, Pakistan — and Afghanistan. These strong affiliations have survived largely undisturbed, at least until recent times, under the policy of successive Pakistani governments that have allowed the tribal areas to operate largely outside the ambit of Pakistani law.