Three major militant groups in Pakistan's troubled northwest have put aside their differences and pledged support to the main insurgent group confronting a full-scale military offensive, commanders said on Thursday.

Pakistan's military has stepped up its fight against militants since Taliban gunmen massacred more than 150 people, most of them children, at a school in Peshawar in December.

On Thursday, leaders of Taliban splinter group Jamat-ul-Ahrar and militants belonging to Lashkar-e-Islam pledged allegiance to Tehreek-e-Taliban Pakistan (TTP), which claimed responsibility for the deadly school shooting.

TTP is led by Mullah Fazlullah, who is believed to have ordered the 2012 shooting that gravely injured Pakistani schoolgirl Malala Yousefzai, last year's Nobel Peace Prize laureate.

"We congratulate the Ummaat-e-Muslima (Muslims across the world) and especially the Mujahideen (holy warriors) of Pakistan for the coalition of strong Jihadi groups," Ehsanullah Ehsan, a Jamat-ul-Ahrar spokesman, told AFP in an emailed statement.

Lashkar-e-Islam, led by warlord Mangal Bagh, has also joined the alliance, Ehsan said.

The group is feared for kidnappings and extortion in Khyber, one of seven tribal districts along the Afghan border.

Jamat-ul-Ahrar split from TTP last year but has claimed many attacks against Pakistani troops and Shiite Muslims.

TTP main spokesman Muhammad Khorasani confirmed the re-alliance in a separate statement sent via email.

Both the spokesmen threatened to carry out more attacks against Pakistani troops "jointly".

A Taliban commander in the country's northwest told AFP that representatives of the three militant groups met somewhere in eastern Afghanistan and decided to align after holding several meetings.

Pakistani troops have been engaged in a full-scale offensive against Taliban and other militants in North Waziristan and Khyber tribal districts along the Afghan border since last year.

The school massacre, Pakistan's deadliest ever terror attack prompted the government to announce a tough crackdown on militant groups.

A moratorium on executions in terror cases was lifted and the constitution was amended to set up military courts for the speedy trial of terrorism cases.

The government extended the order on Tuesday, directing provincial governments to proceed with hangings for all death row prisoners who had exhausted their appeals and clemency petitions.

Twenty-four people have been hanged since executions were restarted in December.