The death toll from a suicide bombing in Pakistan that targeted a hard-line religious group's political rally and raised security concern for a general election due by November has risen to 56, a government official said on Tuesday.

The attack was the deadliest on a political party since campaigning for the last election, in 2018, in the nuclear-armed South Asian nation of the 220 million people.

Militant violence also marred preparations of a 2013 general election.

The toll from Sunday's attack in the northwestern district of Bajaur, on the border with Afghanistan, could rise, said Anwar-ul-Haq, the region's deputy commissioner.

The Islamic State militant group claimed responsibility for the blast at a gathering of the Jamiat Ulema Islam-Fazl (JUI-F) party, which is known for its links to hardline Islamists but which condemns militants seeking to overthrow the government.

The JUI-F believes in a democratic, parliamentary system while also being a strong supporter of the Afghan Taliban who are ideological opponents of the Islamic State group.

The firebrand cleric leader of the JUI-F, which is allied with Prime Minister Shehbaz Sharif's ruling coalition, condemned the attack. "The JUI is a proven force on the political front," the party leader, Fazl-ur-Rehman, said in a video statement.

"Such violence can't stop us," he said, adding that leaders had to come together to find a way to end the violence.

Prime Minister Sharif denounced the blast, which came after months of political tension and an economic crisis, as an attack on the democratic process.

Islamic State's Amaq news agency said in a claim for the attack on its Telegram channel that democracy was hostile to Islam.

"The attack comes in the natural context of the ongoing war waged by the Islamic State against 'democracy' as a regime hostile to true Islam and in conflict with its divine law," the news agency said.