As many as 140 people may have been killed in the attack, sources close to the situation told CNN. The sources are officials who requested anonymity so as not to interfere with the Ministry of Defense or Ministry of Interior, which are responsible for reporting official tolls.
Zabiullah Mujahid told CNN in an email Saturday that the attack was undertaken because pro-Afghan government forces killed two Taliban shadow governors in the northern provinces of Kunduz and Baghlan.
The assault began as soldiers were observing Friday prayers at Camp Shaheen near Mazar-e Sharif
, one of the nation's most populated and developed cities, the Afghan military said. The city is in Balkh province and the base is the headquarters of the 209th Shaheen Corps.
Dawlat Waziri, an Afghan Ministry of Defense spokesman, declined to give a more precise accounting of dead and wounded but said one would be released later.
Taliban fighters dressed in military uniforms raided the army base in northern Afghanistan, raking it with gunfire in an hours-long attack that left more than 100 soldiers dead or wounded, officials said.
The uniformed attackers entered the base in vehicles and opened fire, Afghan army spokesman Abdul Qahar Araam said. The gunfire was followed by an explosion at one of the base's gates.
The attack lasted six hours. By the end, at least five attackers were killed and one was arrested, Araam said.
Afghan President Ashraf Ghani has declared Sunday to be a national day of mourning in remembrance of the Afghan forces killed on the base. The Afghan national flag will be flown at half-staff in Afghanistan and its missions.
Breaking a 'stalemate'
The massive attack is a setback for the Afghan government and its coalition allies faced with a persistent Taliban insurgency
and the presence of ISIS
and other terrorist groups.
US troops have been fighting there for nearly 16 years against a resilient Taliban.
In February, Gen. John Nicholson
, commander of US forces in Afghanistan, told the Senate Armed Services Committee that leadership assesses "the current security situation in Afghanistan as a stalemate."
"We remain concerned about multiple critical factors," he said.
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.
There are 8,400 US troops in Afghanistan and 6,000 troops from NATO and allied countries. Nicholson said the coalition faces "a shortfall of a few thousand troops" to break the "stalemate."
From January 1 through November 12 last year, 6,785 Afghan national security forces were killed, according to the latest quarterly report
of the special inspector general for Afghanistan reconstruction.
The agency's analysis of information from US forces in Afghanistan "suggests that the security situation in Afghanistan has not improved this quarter."
"The numbers of the Afghan security forces are decreasing, while both casualties and the number of districts under insurgent control or influence are increasing," according to the January 30 report to Congress.
Congress created the office "to provide independent and objective oversight of Afghanistan reconstruction projects and activities."
The Taliban action comes more than a week after the US military dropped America's most powerful non-nuclear bomb
on ISIS targets in Afghanistan, killing 94 ISIS fighters.
ISIS claimed responsibility
for an attack last month on Kabul's heavily fortified diplomatic quarter.
National security adviser Lt. Gen. H.R. McMaster
recently visited Afghanistan as Washington considers a full "strategy review." of policy toward that country.
"In recent years, at a period of our maximum effort, we didn't have as reliable a partner in the Afghan government as we would've liked," McMaster said. "Now we have a much more reliable Afghan partner and we have reduced considerably the degree and scope of our effort."
Taliban leader killed in Kunduz
US forces in Afghanistan on Saturday announced another incident in the northern part of the country, saying Taliban leader Quari Tayib was killed in an airstrike in Kunduz province.
The US military said Tayib was once known as the Taliban shadow governor of neighboring Takhar province.
The strike was part of ongoing efforts to deny Taliban freedom of movement in the area, the military said.
Tayib had been a target of interest since 2011 and was directly responsible for the deaths of US service members in Afghanistan, the military said.
A compound owned by Tayib was targeted in the strike, which also killed eight other Taliban fighters.