Kabul - Six Afghan Taliban inmates on death row were hanged on Sunday, government sources said, in the first set of executions endorsed by President Ashraf Ghani since he came to power in 2014.

Ghani last month vowed a new hardline stance against the Taliban after an insurgent attack killed 64 people in Kabul, in what appeared to be the deadliest attack on the Afghan capital since 2001.

The executions occurred despite a threat of reprisal attacks from the Taliban , which launched its annual spring offensive last month, widely expected to be the worst fighting season in 15 years.

"In accordance with the Afghan constitution... Ghani approved the execution of six terrorists who perpetrated grave crimes against civilians and public security," the presidential palace said in a statement.

"This order has been carried out today after... considering the human rights obligations of Afghanistan ... and in accordance with Afghan laws." A government source told AFP that all six were Taliban inmates , most of them convicted of insurgent strikes that resulted in mass casualties.

There was no immediate comment from the Taliban .

In an unusually vitriolic speech last month, Ghani pledged a tough military response against the Taliban and vowed to enforce legal punishments, including executions of convicted militants.

His remarks were in response to a brazen Taliban assault on April 19 on a security services office in the heart of Kabul, seen as the opening salvo in this year's Taliban spring offensive.

The carnage left 64 civilians and military personnel dead and cast a pall over international efforts to jumpstart Pakistan-brokered peace talks, which stalled last summer after the Taliban belatedly confirmed the death of longtime leader Mullah Omar.

Ghani also threatened diplomatic reprisals against Pakistan if it refuses to take action against insurgent havens on its soil.

Ghani's remarks reflected his frustration after he expended substantial political capital since coming to power in 2014 in courting Pakistan in the hope of pressuring the militants to the negotiating table.

The Pakistani government recently admitted, after years of official denial, that the Taliban leadership enjoys safe haven inside the country.

Following Ghani's speech, the insurgents had threatened "grave repercussions" if Taliban inmates were executed.

"If the enemy (Kabul) decides to carry out executions, the Islamic Emirate (Taliban) will respond with everything within its power to defend the oppressed nation," the insurgent group said in a statement on its website late last month.

"The enemy's supposed judicial bodies could once again pay a hefty price for their crimes."

The Taliban announced the start of their spring offensive on April 12, dubbed "Operation Omari" in honour of Mullah Omar, vowing large-scale attacks across Afghanistan .

The Islamists, who have been waging an insurgency since being toppled from power in 2001, also promised attacks on the 13,000 NATO troops stationed in the country, officially in a training and advisory role since the end of their combat mission in 2014.