A blast killed 24 people and injured 70 in the north western Pakistani city of Parachinar on Sunday, officials have said, and a banned Sunni Islamist group has claimed responsibility for the attack.

Parachinar local government official Riaz Hussain said Sunday's blast targeted crowds at a market.

"The death toll has reached 24 and 70 are wounded," said Amjad Ali Khan, the political head of Kurram Agency. Parachinar is the capital of Kurram.

Officials said the death toll could still rise because of the number of people who are seriously wounded.

"This is revenge for the killing of Muslims by the Syrian president and Iran," said Ali bin Sufyan, spokesman for Lashkar-e-Jhangvi (LeJ), an Islamist group whose sectarian ideology is closely aligned with Islamic State.

Earlier this week Reuters reported that a Shiite unit of Pakistani fighters known as the Zeinabiyoun were joining the war against Islamic State in Syria.

Many come from Parachinar, which has a large Shiite population, unusual in Sunni-majority Pakistan.

Regional sources said there were hundreds of Pakistanis fighting in Syria.

Mr Sufyan said Parachinar residents should not travel to Iran or Syria to fight in the war there on behalf of the Shiites.

The explicit linkage of the Pakistan bombing to the war in Syria will alarm Pakistani officials, who are keen to play down reports that Islamic State is seeking to establish a foothold in their country.

A couple of small militant groups who splintered from the Pakistani Taliban pledged allegiance to Islamic State, but they have not had much impact.

Ex-Afghan Taliban militants pledge to IS

But across the border in Afghanistan, militants who split from the Taliban and pledged allegiance to Islamic State have established training camps, taken over territory and clashed with the Taliban.

Like Islamic State, which set up a cross-border caliphate in parts of Iraq and Syria, the LeJ wants to kill or expel Pakistan's minority Shiites and establish a Sunni theocracy.

The LeJ had once enjoyed the open support of Pakistan's powerful spy agency, the Directorate for Inter-Services Intelligence, but the group suffered a series of setbacks this year.

The leader of LeJ, his two sons and several top officials were shot dead in police custody this July; police say they were trying to escape.

Last month, another top LeJ commander was shot by police while in custody.