Pakistan said Tuesday it has killed 21 militants in air strikes and ground operations that began overnight in a northwest border region where it is carrying out an offensive to clear Taliban strongholds.

Military spokesman Asim Bajwa said the strikes, which took place in the Shawal Valley of North Waziristan tribal district, began Monday night and continued into Tuesday.

"Ops continue. Imp heights & passes along Pak-Afgn Bdr secured. Valley's sanitisation in progress," he said on Twitter.

He later posted photographs of soldiers fanning out in a forested area and a ruined building engulfed in smoke.

It was not possible to confirm how many people were killed in the offensive, nor whether they were indeed combatants, as journalists are denied access to the conflict zone.

Pakistan began its operation to clear Taliban and Al-Qaeda strongholds in North Waziristan in 2014, and claims to have killed more than 3,750 militants since then with no civilian casualties.

But critics and rights activists say there is no transparency around the offensive and no clear end in sight.

In late February the army said it was entering the final phase of the operation, but offered no further details.

Meanwhile, the toll from a deadly Taliban suicide bombing on a court complex in northwest Pakistan rose Tuesday after a victim died overnight, officials said.

"The death toll from yesterday's blast has risen to 18 as one civilian succumbed to his injuries while two injured are in critical condition," Tariq Hassan, a senior local administration official, told AFP.

The attack occurred Monday in Shabqadar town and was claimed by the Pakistani Taliban's Jamat-ul-Ahrar faction, which said it was to avenge the hanging of Islamist assassin Mumtaz Qadri.

Qadri was feted as a hero by Islamists after he gunned down the liberal governor of Punjab in 2011 over a call to reform the country's blasphemy law.

He was hanged on February 29 in what analysts described as a key moment in Pakistan's long fight against militancy, saying it demonstrated the government's resolve to uphold the rule of law rather than allowing extremism to flourish.

His funeral brought up to 100,000 people on to the streets, hailing him as a hero.