The Afghanistan President Ashraf Ghani declared a national day of mourning after scores of soldiers were killed by Taliban fighters disguised as fellow soldiers, in the deadliest attack of its kind on an Afghan military base.

The defence ministry has said more than 100 died or were injured in the Friday attack in the northern city of Mazar-i-Sharif, although no exact numbers have yet been released.

One official, speaking on condition of anonymity, told Reuters news agency that at least 140 soldiers were killed with many others wounded and some officials stating that the figure may be even higher.

The attack starkly highlighted the difficulty of the long struggle by the Afghan government and its international backers to defeat the Taliban insurgency.

After arriving in Mazar-i-Sharif to visit the base on Saturday, Ghani ordered that flags be flown at half mast on Sunday in memory of the troops who died. President Ghani has called for a “serious” investigation into the attack.  In a statement online, he condemned the attack as “cowardly” and the work of “infidels”.

As many as 10 Taliban fighters, dressed in Afghan army uniforms and driving military vehicles, made their way into the base and opened fire on mostly unarmed soldiers eating and leaving a mosque after Friday prayers, according to officials.

They used rocket-propelled grenades and rifles, and several detonated suicide vests packed with explosives, officials said.

Taliban spokesman Zabihullah Mujahid said on Saturday the attack on the base was retribution for the recent killing of several senior Taliban leaders in northern Afghanistan.

The U.S. military command in Kabul said an American air strike had killed a commander, Quari Tayib, and eight other Taliban on April 17.