KABUL, Afghanistan – A Taliban suicide bomber rammed a motorcycle packed with explosives into a joint U.S.-Afghan patrol on Monday, killing 14 people including three Americans in the latest attack on an increasingly fraught program to help Afghan forces take over security so foreign troops can withdraw from the country over the next two years.

The attack followed more American casualties over the weekend that pushed the U.S. military's death toll for the 11-year-war above 2,000 – a figure that has climbed steadily in recent months as attacks on the so-called "partnering" initiative have risen.

Joint patrols between NATO and Afghan forces, like the one targeted Monday, have been limited following a tide of attacks by Afghan soldiers and police on their international allies. Last month, the U.S. military issued new orders that require units to get approval from superiors before conducting operations with Afghans. Two weeks later, U.S. officials said most missions were being conducted with Afghans again, though the system of approvals remained in place.