Pakistan’s powerful military chief has ordered his troops to begin the "last phase" of a bloody operation targeting militants in the restive northwest along the Afghan border.

Pakistan's powerful military chief has ordered his troops to begin the "last phase" of a bloody operation targeting militants in the restive northwest along the Afghan border, the army said.

General Raheel Sharif gave the order during a visit to forward troops carrying out the offensive in North Waziristan, in which the army says it has killed thousands of militants since it began in June, 2014.

"General Raheel Sharif directed to commence forthwith the last phase of operation in North Waziristan which aims at clearing the remnants of terrorists from their hideouts in deeply forested ravines, isolate them and indiscriminately sever their links with their abettors anywhere across the country," the army said in a statement late Wednesday.

A later tweet from a military spokesman said the operation "has begun", though the army offered no further details.

The army launched Operation Zarb-e-Azb under US pressure in 2014 in a bid to wipe out militant bases in the North Waziristan tribal area and bring an end to the near decade-long insurgency that has cost Pakistan thousands of lives.

The operation was intensified after the Taliban massacred more than 150 people, the majority of them children, at a school in the northwestern city of Peshawar in December 2014.