Former Afghan Taliban chief Mullah Mansour was tracked down by United States (US) spy agencies when he was visiting his family in Iran, the Wall Street Journal reported on Wednesday.

Iran, however, has denied that Mansour entered Pakistan from the Islamic republic.

The WSJ report claims US surveillance drones don't operate in the Iran-Pakistan border area, due to which intercepted communications and other intelligence allowed agencies to track Mansour down and lay a trap for him as he crossed the border into Balochistan in a white Toyota Corolla.

Armed drones were sent across the Afghan border to lock on to the car and take down the target, officials told WSJ, before the Taliban leader could reach Quetta.

The WSJ said the killing of Mansour in Balochistan was a warning to Pakistan that the US would take action on Pakistani soil without prior notification if necessary.

US President Barack Obama and State Department Spokesman Mark Toner have both said American forces will continue carrying out strikes on Pakistani soil to "remove terrorists who are actively pursuing, planning and directing attacks against US forces".

Toner during a State Department briefing denied having knowledge of where exactly Taliban chief Mullah Mansour was eliminated. "I don’t have any more clarity of where the actual strike took place. What I can say was in that border region. I just can’t say on which side of the border it was," he said.

An earlier Dawn report said the US believes Mullah Mansour was a major impediment to peace talks between the Afghan government and the Taliban.

After joining the peace process in Murree in July 2015, the Taliban refused to return to the negotiating table when it emerged that former leader Mullah Omar had been long dead.

Pakistan security officials, however, believe that Mansour was not averse to peace talks but was in the process of consolidating his position in order to convince the rank and file to resume negotiations.

"Eighty per cent of his people were against the talks. Only 20pc favoured the peace process," one official said. "He was trying to consolidate to neutralise his opponents and prevail upon them to hold talks," he said, adding "there was a lot of division within the Taliban leadership on the peace process".

A Taliban delegation, the official said, was already in Pakistan to discuss the prospects and terms and conditions for talks.

"Things would have been much clearer in July," he noted. "But now it seems that the peace process has been pushed away by another six months to a year. Bad timing."