The attack came hours after Mattis arrived in the Afghan capital, the first member of Donald Trump's cabinet to visit the war-torn country since his pledge to stay the course in America's longest war.The unannounced high-level visit comes as Afghanistan's beleaguered security forces struggle to beat back the Taliban, which has been on the offensive since the withdrawal of US-led NATO combat troops at the end of 2014.
Mattis, along with NATO chief Jens Stoltenberg, will meet President Ashraf Ghani and other top officials to discuss the US-led NATO "train and assist" mission designed to strengthen Afghanistan's military so it can defend the country on its own.
"Discussions will focus on the NATO-Afghanistan partnership, including the ongoing NATO-led Resolute Support Mission in support of the Afghan National Defence and Security Forces," a statement on Resolute Support's Facebook page said.
US generals have for months been calling the situation in Afghanistan a stalemate, despite years of support for Afghan partners, continued help from a NATO coalition and an overall cost in fighting and reconstruction to the United States of more than $1 trillion.
The war turns 16 in October and America is pressing NATO partners to increase their own troop levels in the country to help Afghan forces get the upper hand in the grinding battle against the Taliban and ISIS.
The resurgent Taliban have promised to turn Afghanistan into a "graveyard" for foreign forces and have been mounting deadly attacks as they maintain their grip on large swathes of the country.
As of February only about 60 percent of Afghanistan's 407 districts were reported to be under government control, according to the US watchdog agency SIGAR.