A suicide bomber drove a motorcycle into a joint convoy of U.S., NATO and Afghan forces on Monday, blowing himself up and killing six Americans, a brutal sign of deteriorating security in a region where Western troops are trying to help locals overtake the Taliban.

The explosion marked the deadliest day for American troops in Afghanistan since an October helicopter crash, and the worst attack on Americans since six died in a July 2012 roadside bombing, according to the website iCasualties.

The Taliban posted a message on Twitter claiming responsibility for Monday's attack, which officials said targeted a routine security patrol outside Bagram Airfield north of Kabul around 1:30 p.m. local time.

Another two Americans and an interpreter were injured, U.S. officials said.

The attack came days after the Pentagon reported that efforts to train and conduct counter-terrorism operations with Afghan forces had grown tougher, with insurgent attacks and casualties on the rise.

President Obama acknowledged those troubles in October, when he announced that he would keep nearly 10,000 American troops in Afghanistan through most of 2016. Handing security over to Afghan forces completely was too much of a risk, Obama said, especially given attempts by ISIS to move into the region.

The White House press office released a statement Monday evening expressing condolences to victims' families and promised not to waver in its plan "to promote peace and stability in Afghanistan."