KABUL (Reuters) - A suicide bomber killed eight American civilians in an attack at a military base in southeastern Afghanistan on Wednesday, one of the highest foreign civilian death tolls in an insurgent strike in the eight-year war.

The explosion was at a military base in Khost province, near the border with Pakistan, U.S. officials said. The base is a center for civilians working on reconstruction projects, a key part of U.S. President Barack Obama’s strategy to stabilize the country.

“We can confirm that there was an explosion in Khost province and eight Americans have been killed,” said one U.S. official in Kabul.

Some people, none of them U.S. or NATO troops, were wounded in the explosion at Forward Operating Base Chapman, defense officials said, without providing any further details.

Asked whether the suicide blast occurred inside the base, one official said: “That’s my understanding.” Another senior official confirmed the attack involved an explosive vest.

Attacks in Afghanistan this year have spiraled to their highest levels since the Taliban were overthrown by U.S.-backed Afghan forces in late 2001.

Civilian and military casualty tolls have reached record levels this year, with suicide attackers even targeting United Nations employees at a guesthouse in Kabul, killing five and wounding several others.

After that incident, U.N. Secretary-General Ban Ki-moon said Afghanistan and neighboring Pakistan were the most dangerous places for United Nations civilian staff.

Many civilians working outside Kabul have retreated into army bases as the security situation has deteriorated. Bases are heavily fortified and require extensive security checks to enter.

Foreign aid agencies warned earlier this year that the shift into the military bases, and the use of military personnel to carry out development projects, risked a dangerous blurring of the boundaries between troops and civilians.

Washington is sending 30,000 extra troops to Afghanistan in an attempt to stem the violence, with NATO allies also contributing thousands more.

The Obama administration has pledged a “civilian surge” in Afghanistan, adding hundreds of U.S. experts to support work on development projects that aim to undermine support for the Taliban and other insurgents.

Khost is one of the areas of Afghanistan where the Taliban insurgency is strongest, and most foreigners there are troops or working under military protection.

In September a suicide bomber rammed a car into a military convoy of foreign forces there, killing one American.

Afghan civilian casualties have also been increasing, raising tensions between the Afghan government and Western forces.

Hundreds of Afghans took to the streets on Wednesday to protest a raid by foreign troops which Afghan officials say killed 10 civilians. NATO forces say the deaths occurred in a battle in which nine insurgents were killed.