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.
The Monday morning explosions came as the Taliban and the Afghan government are expected to launch much delayed peace talks aimed at ending the country's nearly two-decade-old war.
"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 the 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.
It said such "indirect-fire incidents" cause thousands of deaths every year.
The Taliban have denied responsibility for the explosions, which also wounded 15 civilians, and accused the Afghan army of firing the mortars.
The insurgents hold large areas of Helmand province, a region known for its opium production, where Afghan forces and the militants engage in almost daily fighting.
Violence dropped after the Taliban offered a rare ceasefire to mark the Eid festival on May 24, but officials say the militants have now stepped up attacks against security forces and civilians.
In recent weeks, several brutal attacks targeted Afghan security forces, mosques, human rights workers and prosecutors.
The Taliban have denied most of these attacks, but acknowledge their fighters have been targeting security forces across most provinces.
The rise in violence comes as both the government and Taliban are set to engage in peace talks.
The United States has pushed for launching the talks after it signed a deal with the Taliban in February that paves the way for withdrawing all foreign forces by the middle of next year.
The Taliban's chief negotiator Mullah Abdul Ghani Baradar reaffirmed the group's commitment to the deal with Washington during a call with US Secretary of State Mike Pompeo on Monday.
Pompeo acknowledged the Taliban had refrained from attacking urban centres and military bases under the deal, but called on them to do more to reduce overall violence, according the group's spokesman Suhail Shaheen.
"We are committed to starting intra-Afghan talks," Baradar told Pompeo, blaming the hold-up on the delayed release of Taliban prisoners, according to Shaheen.
The Taliban insist negotiations will begin only after Kabul completes the release of 5,000 of the group's militants held in Afghan jails as agreed in the deal with Washington.
Afghan authorities have so far released about 4,000 Taliban.