Islamabad, Pakistan — Fighting between the Pakistani military and pro-Taliban militants has forced 20,000 people to flee from Pakistan to war-torn Afghanistan.

The United Nations said that an intensive military operation in the border tribal area of Bajaur had led to thousands of refugees crossing into the Afghan province of Kunar.

“More than 3,900 families, or around 20,000 individuals, have fled fighting in Bajaur into Kunar province in eastern Afghanistan,” the U.N. High Commission for Refugees in Afghanistan said.

The Pakistani military launched an offensive in August in Bajaur where American intelligence believed that Al Qaeda’s no. 2 in command, Ayman al-Zawahiri, had taken sanctuary and militants had set up a parallel government.

The majority of an estimated 300,000 people who have escaped the conflict have moved to areas within Pakistan. But the fighting is edging slowly toward Bajauri districts closer to the Afghan-Pakistan border.

“In the last two weeks alone, over 600 Pakistani families have fled into Afghanistan,” the United Nations said in a statement.

Around 9,000 Pakistani soldiers are deployed in Bajaur and up to 1,000 militants have been killed in clashes this month, according to the army.

Pakistani fighter jets pounded militants’ hideouts in the Bajaur region after the Taliban announced a ceasefire for the Muslim festival of Eid-al-Fitr, later this week. Ten militants were reported to have died in the assault.

The porous nature of the disputed border has been underscored as Pakistan and Washington continued to trade blows over their combined inability to stem the cross-border infiltration of militants.

An American commander said that the Bajaur operation had not yet led to a significant decrease in infiltration from Pakistan to Afghanistan.

However, Pakistani commanders have described how militants fighting in Bajaur are receiving weapons and reinforcements from Afghanistan.

There were fears that thousands of people who had fled from Pakistan are living without shelter despite cross-border tribal links.

“They have mainly been provided accommodation by relatives and friends,” a UNHCR spokesman, Nadir Farhad, said. He added that some 200 families are already living without shelter.

In the past, millions of refugees crossed the other way, to escape from violence in Afghanistan. Some 4 million Afghans fled the civil wars of the 1980s and ’90s seeking refuge in Pakistan. More than half have now returned.