KABUL (Reuters) - Two NATO helicopters killed 30 insurgents on Pakistani soil after a rare manned pursuit across the border from Afghanistan, NATO forces said Sunday.

The two Apache attack helicopters from the NATO-led International Security Assistance Force (ISAF) crossed the border from eastern Afghanistan Friday after the insurgents attacked a remote Afghan security outpost in Khost province.

ISAF spokesman Sergeant Matt Summers confirmed that the helicopters had attacked after crossing into Pakistan. He did not comment on which countries’ forces were involved, but the United States is the only coalition member that uses Apaches.

The United States regularly uses pilotless drone aircraft for missile strikes in Pakistan’s borderlands, known to be a haven for militant groups including the Afghan Taliban who launch cross-border attacks from Pakistan’s northwest.

However, manned military flights across the border are extremely rare.

ISAF said in a statement issued late Sunday that the helicopters were following its rules of engagement when they crossed into Pakistan.

Two Kiowa helicopters returned to the area Saturday and killed at least four more insurgents, the statement said.

ISAF was not immediately able to confirm whether the Kiowa helicopters had also crossed the border. U.S. forces make up the majority of ISAF troops in Afghanistan’s east.

The drone flights have caused resentment in overwhelmingly Muslim Pakistan, where anti-American feeling runs high.

U.S. officials say the pilotless drones are one of the most effective weapons against militants. The strikes have killed senior Taliban and al Qaeda figures.

A new book by veteran U.S. journalist Bob Woodward claims that the Central Intelligence Agency also has a 3,000-man “covert army” of elite, well-trained Afghans, who conduct secret operations in Pakistan, according to the Washington Post.

The newspaper obtained an advance copy of the book, which is due for release Monday.

Despite the presence of almost 150,000 foreign troops, violence is at its worst across Afghanistan since U.S.-backed Afghan forces ousted the Taliban in late 2001.

The Taliban-led insurgency has spread out of traditional strongholds in the south and east into once relatively stable areas in the north and west.

Apart from the Afghan Taliban, the al Qaeda-linked Haqqani network also operates extensively in Afghanistan’s east and has launched a series of bold attacks against foreign forces and Afghan government targets in the past year.