JALAW GIR, Afghanistan (AP) -- The slaughter of 11 sleeping Chinese road workers Thursday was the deadliest attack on foreign civilians since the fall of the Taliban, and dealt a blow to U.S. claims that Afghanistan is becoming safer ahead of milestone elections this fall.

The assault in the relatively tranquil north also underlined the dangers for thousands of foreigners helping to rebuild Afghanistan, where President Hamid Karzai's U.S.-backed government is fighting off an insurgency by Taliban rebels and their al-Qaida allies. Aid workers warned the bloodshed could prompt a further pullback of their activities to the capital, Kabul.

The United Nations condemned the "cold-blooded" attack in Kunduz province and halted registration of voters there until at least Saturday -- a further setback in preparations for the September elections, with still only one-third of the estimated 10 million eligible Afghan voters signed up. It also told staff to stay off the roads.

Although on a smaller scale, attacks on foreign civilians have intensified in Afghanistan since a similar pattern of targeting expatriates emerged in Iraq.