Shahbaz Sharif (3R) meet a Bajaur blast victim along with the Pakistan's army Chief General Syed Asim Munir (2L) at a military hospital in Peshawar. AFP

Shahbaz stopped short of accusing Afghanistan's Taliban government of knowingly allowing attacks from its soil, but he did say Pakistan militants were operating from "sanctuaries" in the neighbouring country.

The prime minister asked Afghanistan's Taliban rulers to do more to prevent militants from crossing the border to stage attacks such as the massive suicide bombing earlier this week that killed dozens in Bajaur district.

The appeal came days after the bomber struck an election campaign rally of supporters of the Jamiat Ulema-e-Islam-F (JUI-F) political party workers on Sunday, killing dozens and wounding scores in the district of Bajaur.

The death toll from the bombing rose to 63 on Wednesday, after more critically wounded persons died in hospitals in Bajaur and the northwestern city of Peshawar while 123 are still under treatment, hospital and government officials said.

"I can confirm that so far 63 people have died in the suicide bombing,” Liaquat Ali, a spokesman for the state-run hospital in Bajaur, said on Wednesday. He said some of the wounded people are still being treated at hospitals in Peshawar, the capital of Khyber Pakhtunkhwa province.

He said some of the wounded were still not in stable condition. An Afghan-based branch of Daesh group has claimed responsibility for the Bajaur attack.

Shahbaz said the militants find sanctuaries inside Afghanistan, regroup and rearm there, and subsequently infiltrate Pakistan where they carry out anti-government attacks.

The Afghan Taliban government "should undertake concrete measures toward denying their soil be used for transnational terrorism,” he said while visiting some of the wounded in a Peshawar hospital.

Islamabad has previously said fighters from the Pakistan Taliban were operating freely from Afghanistan — a charge Kabul routinely denies.

"The Prime Minister noted with concern the involvement of the Afghan citizens in the suicide blasts," a statement from Shahbaz Sharif's office said.

It noted there was "liberty of action available to the elements hostile to Pakistan in planning and executing such cowardly attacks on innocent civilians from the sanctuaries across the border."

Since the Taliban surged back to power in Afghanistan two years ago, Pakistan has witnessed a dramatic uptick in militant attacks focused in its western border regions.

Taliban authorities have consistently pledged not to let Afghan territory be used by foreign militants to stage attacks — a key part of the accord that saw US-led forces leave after a 20-year occupation.

Afghan government spokesman Zabihullah Mujahid told AFP on Wednesday that the Khar attack was a "criminal act." "Such incidents should be prevented where they are happening and being coordinated," he said.

"The Islamic Emirate of Afghanistan is very serious about preventing its soil being used against anyone, and we won't allow anyone to create a sanctuary here."

The United States condemns "in the strongest terms" the suicide bombing in Pakistan on Sunday that killed dozens of people, State Department spokesman Matthew Miller said on Tuesday.

"No country should have to suffer such acts of terror," Miller told a briefing. The death toll from the attack, which targeted a hard-line religious group's political rally, has risen to 56, a government official said on Tuesday.

Agencies