Escalating violence in Pakistan's lawless tribal border regions has forced thousands to flee to eastern Afghanistan, the UN reported Monday.

The United Nations is reporting that 20,000 Pakistani refugees have fled the northern tribal region of Bajur to Afghanistan's Kunar province to avoid increased fighting.

Pakistan's military is carrying out an offensive in Bajur, a region mostly controlled by militants opposed to the Afghan and Pakistani governments.

Tens of thousands of civilians are also believed to have fled to other parts of Pakistan as a result of the two-month-old offensive. The UN's refugee agency said most of them are expected to return home after the fighting ceases.

"In the last two weeks alone, over 600 Pakistani families have fled into Afghanistan," the UNHCR said. "While the vast majority of them are living with their relatives and friends, there are already some 200 families who live in the open air."

According to Pakistani officials, the fighting in Bajur — the most northerly of Pakistan's wild tribal regions — has displaced as many as 500,000 people. Most have found shelter with relatives across northwestern Pakistan, though about 100,000 have taken refuge in camps set up by Pakistani authorities.

The UN report comes as new statistics provided by the Pakistani military shows that suicide attacks in the country have killed nearly 1,200 people in the last 15 months. The statistics show there were a total of 88 suicide bombings in that period, leading to 1,188 deaths, 847 of which were civilians. The rest were troops and police.

In all, the military said 1,368 members of the security forces had died since 2001, when former President Pervez Musharraf sided with the United States in its war on terror.

15 insurgents killed in overnight attack

In the latest round of violence, Pakistani forces have killed 15 insurgents in ongoing clashes with Taliban and al-Qaeda militants along the border with Afghanistan, officials reported Monday.

In the overnight attack, Pakistani troops repelled 50 militants north of Khar in Bajur, said a local police official. Militants also attacked paramilitary troops near Tang Khata, the official told the Associated Press.

Pakistani officials estimated 15 militants were killed in the clash and more than 12 were wounded.

There is no word of casualties on the Pakistani side.

The army claims to have killed more than 1,000 militants in the two-month-old offensive in Bajur and lost more than 60 troops. It has declined to estimate casualties among civilians.

U.S. officials have praised the operation in Bajur.

The region along the border is a known haven for al-Qaeda and Taliban militants and Pakistani extremists who attack American and NATO troops operating in Afghanistan.

U.S.-led raids across the border from Afghanistan into Pakistani territory have created tension between the two countries.

Pakistani leaders have decried the operations, and vowed the military would defend the country's sovereignty.

Washington has urged Pakistan to assert control in the region and take stiffer action against militants in the mountainous tribal belt. But there's been growing concern in the U.S. that Pakistan is unwilling or incapable of rooting out extremists in its border region.

In his first meeting with Pakistani President Asif Ali Zardari, in New York last week, President George W. Bush said the United States is only trying to help Pakistan protect itself.

Zardari responded that his country is able to find its own solutions to its problems.

With files from the Associated Press