Col. John J. Thomas, U.S. Central Command spokesman, told Washington-based reporters that a Taliban attack on a military base near Mazar-i-Sharif, had caused dozens of "friendly" Afghan casualties.
A Taliban spokesman said in a statement on Saturday that the attack was retribution for the recent killing of several senior Taliban leaders in northern Afghanistan.
The attack is the deadliest in Afghanistan since July 2016, when two Islamic State suicide bombers killed 80 Hazara protesters. "And then they entered the mosque and dining room, killing everyone indiscriminately", Mohammad Hussain, an army officer wounded in the attack, said as he was treated in hospital.
The Taliban claimed responsibility for the assault. The militants had inside help in that attack, security officials said.
"India condemns in the strongest terms the deplorable terrorist attack in Mazar-e-Sharif on 21 April resulting in loss of lives, including the fearless personnel of the Afghan National Defence Forces".
Afzel Hadid, head of provincial council in Balkh told The Associated Press that more than 100 people, both army personnel and others present at the time inside the army crops, were killed in the attack.
Ghani announced that Sunday would be a day of national mourning, with memorial services across the country's mosques and the Afghan flag flying at half-mast, in a statement issued by the Presidential Palace. He said there were probably more than 50 Afghan casualties, but did not say how many of those were killed and how many were wounded.
The base is the headquarters of the Afghan National Army's 209th Corps, responsible for much of northern Afghanistan, including Kunduz, a province which has seen heavy fighting.
That claim has not been confirmed by the Afghan army.
Details were still emerging on Saturday, but other officials said the death toll had been staggeringly high.
"Two suicide bombers detonated their vests full of explosive inside the mosque of the army corps while everyone was busy with Friday prayers", he said. The Afghan national flag will be flown at half-staff in Afghanistan and its missions.
Taliban militants dressed as army men stormed the 209 Shaheen Corps Headquarters, one of the largest military bases in Mazar-e-Sharif city, during Friday prayers.
He cited the government's stability; Afghan military casualties; the influence of Pakistan, Russia and Iran; "the convergence" of various terror groups; the narcotics trade and corruption.