The United Nations Assistance Mission in Afghanistan (UNAMA) on Tuesday blamed Afghan forces for firing mortar shells that struck a busy market in a southern province, killing at least 23 civilians.

"UNAMA's initial impartial findings indicate Afghan National Army mortars inflicted heavy civilian casualties Monday," the mission said on Twitter.

"Multiple credible sources assert that the ANA fired lethal mortars in response to Taliban fire, missing intended target."

Afghan officials had previously blamed the Taliban for Monday's explosions in the Sangin district of Helmand province, with President Ashraf Ghani condemning the incident as a "terrorist attack".

The UN mission called on the Afghan government to set up an independent investigation and urged both the Taliban and government forces to stop fighting in civilian areas.

The Taliban have denied responsibility for the explosions and accused the Afghan army of firing the mortars.

The Monday morning blasts also wounded 15 civilians.