At least 44 people died after a suicide bomber attacked a political convention organized by an Islamist party in northwestern Pakistan, police said.

More than 100 were injured, 17 critically, in the attack targeting members of the Jamiat Ulema-e-Islam-Fazl (JUI-F) party, who had gathered in the town of Khar, close to the border with Afghanistan.

Local police said the attacker detonated explosives near the convention's stage.

There has been no initial claim of responsibility for the attack. But the local branch of ISIS has previously targeted JUI-F party leaders as they consider them apostates.

Pakistani Prime Minister Shehbaz Sharif "strongly condemned" the blast, according to a statement released by the minister's office.

An investigation is underway, and the prime minister's office has requested a report from Interior Minister Rana Sanaullah and the Khyber Pakhtunkhwa government, the statement added.

The leader of the JUI-F party, Maulana Fazlur Rehman, expressed "deep sorrow and regret" following the blast, in a statement released by his press office.

"‎Peace be upon JUI workers," the statement added. "The federal and provincial government should provide the best treatment to the injured."

One witness said more than 500 people were attending the convention when the blast occurred.

"A powerful explosion knocked me unconscious," Rahim Shah told Dawn.com.

When he came around there were "people screaming and even shots were fired," he said, adding that there was blood everywhere.

Pakistani political parties are beginning campaign preparations ahead of elections due later this year.