LONDON — Three U.S. service members were killed by a roadside bomb in Afghanistan Tuesday, marking the worst loss of life this year for Americans in Afghanistan. A total of 18 U.S. service members have been killed there in 2018.

Taliban militants claimed responsibility for the attack in the city of Ghazni, where U.S. forces have waged fierce battles against the Taliban for months. A defense official told CBS News a powerful roadside bomb struck a U.S. military convoy early Tuesday morning. Attack helicopters flew in to support the evacuation of the wounded.

The bombing happened in an area where U.S. Special Forces have been working with Afghan commandos. Last month CBS News joined U.S. forces on the battlefield in Nangarhar Province. The vehicle blown up in Tuesday's attack was similar to one CBS News saw: an MRAP, or Mine Resistant Ambush Protected vehicle.

It's designed specifically to defend against roadside bombs, meaning the explosion that struck the convoy on Tuesday, causing so many casualties, must have been huge.

Of the roughly 14,000 American forces still serving in Afghanistan, most are there to advise and assist Afghan troops who despite 17 years of fighting have continued to lose ground against a resurgent Taliban. The group claimed responsibility for the attack, but they're not the only enemy. While in Afghanistan, CBS News joined U.S. troops in an operation to clear an area of ISIS fighters. A soldier who died over the weekend was on a mission against al Qaeda militants.