Earlier reports stated that gunmen had stormed the bank. Omar Zwak, the spokesman for the Helmand governor, put the toll at 22 dead and 50 injured, but he said those figures were certain to increase as the identification of the remains continued.
On one of its Twitter accounts, the Taliban claimed credit for the suicide bombing in the provincial capital Lashkar Gah, saying it had killed 73 members of the security forces, a figure that conflicts with the official report.
Governor Hayatullah Hayat said by phone that most of those killed were civilians and at least 60 people were wounded.
No terrorist group has immediately claimed responsibility for the attack.More news: Malan leads England to series win over S
The insurgents, believed to control almost 80 percent of the province's countryside, have increasingly been pressing a push onto Lashkar Gah and its environs in efforts to take the city.
"The people and government of Pakistan extend their heartfelt sympathies and deepest condolences to the government and the people of Afghanistan and the bereaved families", the Foreign Ministry said in a statement.
Thousands of civilians and soldiers have died in recent years.
In this undated and unknown location photo, the new leader of the Afghanistan Taliban Maulvi Haibatullah Akhunzadah poses for a portrait.
"The more they insist on maintaining the presence of their forces here or want a surge of their forces, the more regional sensitivity against them will intensify", he said.
Taliban continues to its relentless attacks across the country as the group is accused of causing the majority of the civilian casualties.
Both hostages said they are being treated well by the Taliban but that they remain prisoners and appealed to their governments to help free them.