Thirty-six people, 30 of them children, were killed in an airstrike last month by the Afghan Air Force in northern Kunduz during a religious ceremony, the U.N. revealed in a report on Monday.

According to the U.N. Assistance Mission in Afghanistan (UNAMA) report, the April 2 airstrike in the Dasht-e Archi district on a gathering of men and boys also left 71 people injured, including 51 children.

The report added that UNAMA has credible information that the figures are higher, but the agency's rigorous method of casualty verification requires three independent sources to confirm each death.

The report said that the Quran recitation graduation ceremony was "widely publicized and known," and the crowd was made up primarily of civilians, "many of whom were children, many under the age of 10."

UNAMA added that the high number of child casualties brings into question the Afghan government's respect for rules of war.

Following the attack, the Afghan government had said that the target was a "military gathering" of the Taliban where top Taliban commanders, including members of the Quetta Shura, had gathered.

The defence ministry, which had initially denied civilians were among the dead and wounded, was not immediately available for comment.

"A key finding of this report is that the government used rockets and heavy machine gun fire on a religious gathering, resulting in high numbers of child casualties," UNAMA said.

UNAMA's casualty toll is lower than the original toll of 59 dead and 57 wounded given to AFP by security sources and health officials.

Haji Mohammad Naim, a tribal elder and witness to the attack, had told dpa that at least 50 civilians were killed and at least 65 hurt.

UNAMA's investigators could not confirm if the casualties were all civilians or whether Taliban leaders had been present at the time of the airstrike.

"However, even if the government had a legitimate military target, UNAMA questions the extent to which the government undertook steps and concrete measures to prevent civilian casualties," the report said.

Government officials in both Kabul and Kunduz gave conflicting figures, with some denying any civilians had been killed or that a madrassa had been hit.

The government has sent two teams to conduct an investigation into the incident but so far neither team has "publicly reported their findings", UNAMA said.

While it could not determine if the government had violated international humanitarian law, it called for "further investigation".

"The mission urges the government to investigate, fully document and conduct a transparent review of the circumstances that led to this incident and to take immediate steps to ensure accountability for those responsible along the chain of command," the report said