Afghan police were investigating an apparent army rocket strike on a wedding party that killed at least 28 people, many of them women and children, an official said Thursday, as President Ashraf Ghani marked the country's transition to full sovereignty after the formal end of the 13-year international military mission.

Police in southern Helmand province were looking into how soldiers came to fire a rocket on a house where a wedding was being celebrated late Wednesday, said the deputy provincial police chief, Bacha Gull.

The rocket appeared to have been fired from an army checkpoint near the house in Sangin district as guests waited for the bride to arrive, he said.

Police were "keeping an eye" on two army checkpoints to determine whether the soldiers manning them were engaged in a firefight with Taliban insurgents at the time or whether they fired the rockets arbitrarily. The strike wounded 51 people.

Sangin, in the poppy-producing Helmand River valley, has been the scene of fighting between government forces and Taliban for the past six months, since U.S. forces departed as part of the transition to Afghan sovereignty.

The international mission to rid Afghanistan of insurgents under the leadership of the United States and NATO officially ended on Dec. 31. Afghanistan takes full responsibility for its own affairs from Thursday.

Gull said that funerals, usually held within 24 hours of a death, had been delayed to enable investigators to determine the cause of the rocket strike.

In a televised address, Ghani told Afghans to support their security forces in the interests of building a strong and peaceful Afghanistan.

"If, a year ago today, you had listened to regional and international analysts, they would never have thought that today would happen," Ghani said. "They were thinking how can a country with the problems that Afghanistan has successfully complete a security and political transition."