(CNN) -- Taliban fighters dressed in military uniforms raided an army base in northern Afghanistan, raking it with gunfire in an hours-long attack that killed or injured more than 100 soldiers, officials said.
The attack started as soldiers were observing Friday prayers at Camp Shaheen near Mazar-e Sharif, the country's third-largest city, the Afghan military said.
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.
Dawlat Waziri, an Afghan Ministry of Defense spokesman, declined to give a more precise accounting of dead and injured but said one would be released later.
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.
The base is the headquarters of the 209th Shaheen Corps in Balkh province.