Afghan police stand guard after a suicide bomb attack in Kabul on Nov. 16. (Mohammad Ismail/Reuters)

A suicide bomber detonated a blast Thursday near a political gathering of supporters of one of Afghanistan's most powerful factional leaders, killing at least 12 people in the latest attack in the country's capital.

The Islamic State claimed responsibility in a statement carried by its Amaq News Agency, but the group often asserts links to attacks without offering proof. The Taliban denied any role.

The bomber triggered explosives on his body after police stopped him outside the entrance of the hotel where the Jamiat-i-Islami party members were meeting, police said. The gathering was called to show support for a senior party member, Atta Mohammed Noor, a former militia leader and longtime governor of northern Balkh province.

Atta has criticized the National Unity Government led by President Ashraf Ghani. Abdullah Abdullah, the government's chief executive, is a senior member of Jamiat.

Atta was not at the hotel and had not been expected to attend the function, party officials said. He has made many enemies and been accused of abuses, but he commands a loyal following in the north.

Hours later after the attack, Atta appeared on his private TV channel to say that "some circles within the government" were behind the attack. He did not elaborate.

Tadamichi Yamamoto, the U.N. special representative for Afghanistan, called the attack "totally unacceptable." It was "an act of terror and a serious violation of international law," he said in a statement. "The use of explosive weapons in civilian-populated areas must stop."

A number of former Jamiat militia commanders and a former cabinet minister, Abdul Sattar Murad, were at the event. It was not immediately known whether senior party officials were among those killed.

A security official said the death toll was at least 12. A spokesman for Kabul police said the dead included police officers and civilians, but he gave no figure.

"The function had come to an end, and people were having lunch when the blast was heard," Shamsul Haq Arianfar, a senior Jamiat member, said in a telephone interview. He said that the hotel windows were shattered and that people fled out the back door and through windows, fearing that gunmen would storm the building. He said he saw "bodies being taken away."

It was the second deadly bombing to target a Jamiat event in recent months. In late May, a massive truck bomb exploded in central Kabul, killing more than 150 people, including the son of a party legislative leader. The next day, a suicide bomber attacked his funeral, killing seven people.