Updated

Coalition troops were told to “shelter in place” during a deadly six-hour attack on an Afghan Army base in northern Afghanistan on Friday, as fewer than a dozen Taliban militants massacred nearly 200 troops.

Roughly 30 coalition troops were on the base outside Mazar-i-Sharif, though a U.S. defense official wouldn’t say if Americans were among them.

The group of 10 Taliban members -- including suicide bombers -- launched the surprise attack on Friday, which is prayer day in Afghanistan and other Muslim nations. The time is a “low ops tempo” for the Afghan military, and many Afghan soldiers on base were at a mosque when the attack occurred. Mosque and dining halls were prime targets of the extremists. The coalition typically begins its day around noon on Fridays in the country.

The gunmen and bombers arrived in military vehicles and entered the base disguised as members of the Afghan Army. They began shooting almost immediately after entering the compound in Balkh province. Two attackers died in suicide bombings and the other eight attackers were killed in clashes during the assault.

The attack came just days after National Security Adviser H.R. McMaster visited Afghanistan for meetings with senior U.S. military and Afghan leaders.

The bloody Friday assault continued a trend of heavy losses for Afghan troops. In 2015, Afghan security forces suffered record casualties, with more than 6,500 killed as Taliban militants made territorial gains.

Elsewhere in the country, 300 Marines arrived in Helmand province over the weekend, replacing a U.S. Army unit to train the Afghan Army. It’s the first time Marines have been in Helmand since 2014, when former President Barack Obama declared combat operations over in Afghanistan.