Pakistan's Prime Minister Nawaz Sharif on Monday condemned as "heinous" the alleged execution of 23 kidnapped soldiers by the Taliban, casting doubt over peace talks just two weeks after they began.

A faction of the Tehreek-e-Taliban Pakistan (TTP) from the northwestern district of Mohmand claimed on Sunday night they had killed the paramilitaries whom they seized nearly four years ago near the Afghan border.

"Such incidents have an extremely negative impact on the ongoing dialogue aimed at promoting peace," Sharif said in a statement.

A four-member government negotiating team immediately pulled out of peace talks with their Taliban counterparts scheduled for Monday in the northwestern town of Akora Khattak.

Sharif said Pakistan "cannot afford such bloodshed" and lamented that previous attempts to start dialogue were "sabotaged whenever it reached an encouraging stage".

He announced the start of talks with the Taliban on January 29 to "give peace another chance" following a seven-year insurgency that has claimed nearly 7,000 lives.

Observers say the two sides are far apart on crucial issues and some experts doubt whether Taliban leaders are able to control the entire organisation, particularly the most violent factions.

Excluding the soldiers, some 60 people have died in Islamist-linked violence since the talks began, leading to pressure from some critics for military action against Taliban strongholds in the northwest.

"Our soldiers are being killed and this is the worst thing that can happen to the morale of the troops," defence analyst Talat Masood, a retired general, told AFP.

"It is high time the government takes a firm decision and stands up to protect the lives of the people," he added.

Ayesha Siddiqa, another defence analyst, saw little evidence that the military had the appetite for a sustained campaign and questioned the value of attacking tribal areas which could cause the violence to escalate elsewhere.

"A military operation in the tribal area will eliminate only 40 percent of the terrorists but their major leaders are residing mostly in Punjab and some in Sindh," the respected author on Pakistan's military told AFP.

- Cancelled talks -

Government negotiators declared Monday's scheduled peace talks to be "purposeless" after the "sad and condemnable" murders, while President Mamnoon Hussain expressed "grief and sorrow".

"We regret to say that things are not moving in a right direction," chief government peace negotiator Irfan Siddiqui said in a statement.

He added that a meeting would be convened on Tuesday to discuss the future course of action.

The disappearance of Pakistani soldiers was reported by AFP from the Mohmand tribal district on June 17, 2010.

The paramilitary soldiers from the Frontier Corps vanished after the Taliban attacked their checkpost in a remote area near the Afghan border.

The killing of the prisoners was to avenge the alleged extra-judicial murder of comrades in government custody, the Taliban faction in Mohmand said in its statement sent to AFP.

Peace negotiators nominated by the insurgents said they regretted that Monday's talks in Akora Khattak had been cancelled.

"The two committees should have sat together and it would have enabled them to find ways to prevent the recurrence of such incidents in future," the head of the Taliban committee, Syed Yousuf Shah, said from the town.

"We were close to reaching an understanding on a ceasefire but it is sad that the government committee called off the meeting after considering us as Taliban," he added at a televised press conference.

The Taliban's demands include the nationwide imposition of sharia law, an end to US drone strikes and the withdrawal of the army from northwestern tribal regions -- conditions the government and army are unlikely to be able to meet.

Pakistani troops have for years been battling homegrown insurgents in the tribal belt, which Washington considers the main hub of Taliban and Al-Qaeda militants plotting attacks on the West and in Afghanistan.

A Pakistani security official, talking to AFP on condition of anonymity, denied that insurgents had been killed in custody as claimed by the Taliban in Mohmand.

"The killing of terrorists in the custody of security forces as alleged by the TTP is a baseless allegation and is mere propaganda to justify their dastardly acts of terror," the source said.