ISLAMABAD, Pakistan — A spike in violence along Pakistan's border has driven tens of thousands of people to seek refuge in eastern Afghanistan, the U.N. reported today and new military data showed suicide attacks throughout the country killed nearly 1,200 people in the last 15 months.

Pakistani troops are battling militants who are increasingly using the country's tribal areas as a base for their operations inside Pakistan and also for cross-border attacks into Afghanistan.

The fighting has forced some 20,000 Pakistanis to flee into eastern Afghanistan, the U.N.'s refugee agency reported.

"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."

The UNHCR said it believes that majority of those who have crossed into Afghanistan will return home once the fighting stops.

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.

Pakistan's army claims to have killed more than 1,000 militants in the two-month-old offensive in Bajur. It has declined to estimate casualties among civilians.

Bajur and other tribal regions have fallen largely under the control of militants opposed to the Afghan and Pakistani governments. Seven years on from the Sept. 11 attacks on the United States, the border zone is also still considered a likely hiding place for al-Qaida leader Osama bin Laden.

Rising cross-border attacks on NATO coalition forces have prompted the U.S. military to target the area in recent weeks with a series of missile strikes carried out by drones.

On Sept 3, American commandos launched a highly unusual ground raid that infuriated the Pakistani public and leadership.

Throughout Pakistan, political violence has surged over the last 15 months in response to the July 2007 army siege of militants holed up in Islamabad's radical Red Mosque, where about 100 people died.

A total of 88 suicide attacks have killed 1,188 people, according to new statistics from the Pakistani military. The vast majority of the victims — 847 — were civilians, it said. 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.

Pakistani officials cite such figures to deflect criticism of its record and commitment to the fight against Islamic militants who were once its allies in wars in Afghanistan and Kashmir.

In the latest violence, Pakistani forces killed 15 insurgents in Bajur after they launched an overnight attack, officials said.

Fazl Rabi, a local police official, said Pakistani troops repelled the attack by 50 militants on an army camp about six miles north of Khar, Bajur's main town.

Rabi said militants also attacked paramilitary troops before dawn in the Tang Khata area of Bajur.

Rabi and another government official counted a total of 15 militants killed and more than a dozen wounded in overnight clashes.

They provided no word of casualties on the government side. Poor access and security in Bajur prevents reporters from verifying casualty reports.

However, two intelligence officials said three troops had died in fighting in the past two days. They asked for anonymity because they were not authorized to talk on the record to the media.

Meanwhile, artillery fire directed at suspected militant hide-outs in the Badali area struck two houses and killed two civilians, said Niazur Rehman, a local resident.

Maj. Gen. Jeffrey Schloesser, the top U.S. commander in eastern Afghanistan, which borders Bajur, told The Associated Press last week he was encouraged by Pakistan's offensive, but hadn't yet seen a drop in the number of militants crossing the border.

"We need a persistent series of operations by Pakistan over a lengthier period of time before we see a change there," Schloesser said.