An Islamic State (IS) suicide bomber who killed 64 people at a Shi'ite mosque in northwest Pakistan last week was an Afghan exile who returned home to train for the attack, police said on March 9.
There have been warnings Afghanistan could become a recruiting ground and staging post for militants since the Taliban returned to power last year following the hasty withdrawal of U.S.-led forces.
The Taliban has pledged it will not allow Afghan soil to be used to plot attacks on other nations, but last month the UN Security Council said "terrorist groups enjoy greater freedom there than at any time in recent history."
Two senior Pakistani police officials told AFP that the suicide bomber responsible for the March 4 blast in Peshawar had prepared the attack in Afghanistan.
It was claimed by IS, whose Islamic State-Khorasan (IS-K) affiliate has been active for years in both Pakistan and Afghanistan, where it is in violent competition with the Taliban.
The officials said the attacker was an Afghan national in his 30s who moved to Pakistan with his family decades ago.
"The bomber went (back) to Afghanistan, trained there, and returned without informing his family," one of the senior police officials told AFP.
"Islamic State-Khorasan is becoming a strong threat for us. They are operating from Afghanistan but they have sleeper cells here," he added.
Taliban officials did not immediately respond to requests for comment.
Muhammad Amir Rana, director of the Pakistan Institute for Peace Studies (PIPS), said there was "a lot of apprehension" in the international community over the prospect of Afghanistan becoming a haven for militancy.
While the Taliban can rein in sister groups like Al-Qaeda -- which carried out the 9/11 attacks -- "they cannot guarantee they will do the same about the groups which are not under their control, like IS-K," he said.
"There are a lot of questions on the Taliban's ability to govern Afghanistan," said Rana.
Police said they had killed three "facilitators" of the Peshawar attack in an overnight operation and arrested 20 others suspected of involvement.
IS also claimed responsibility for what it said was a suicide blast on March 8 that killed seven paramilitary troops near a site in southwestern Pakistan where the president had visited less than half an hour earlier.
Since the Taliban's return to power in Afghanistan, Islamabad has acted as a key broker between the hard-liners and the international community.
Pakistan was one of just three nations to officially recognize their first regime from 1996 to 2001.
Its own version of the Taliban, the Tehrik-e Taliban Pakistan, has also staged attacks from hideouts in Afghanistan, testing the diplomatic relationship.
Rana suggested IS-K may be intensifying attacks in Pakistan in order to "increase pressure on the Taliban in Afghanistan" from Islamabad.