A suicide bomber killed at least 20 people in a Friday attack on the Lashkar-e-Islam (LeI) militant group in Pakistan's volatile Khyber Agency region, along the nation's northwestern border with Afghanistan.

The bomber blew himself up amongst a large number of LeI militants in the Nahaqai area of the Tirah valley, a local official told the BBC.

The Taliban claims responsibility for the bombing, saying it was revenge against LeI for gunning down Taliban fighters two months ago. The two hard-line Islamist groups are known to clash as each attempts to establish supremacy in the lawless tribal region.

A LeI spokesman reportedly insisted that only six fighters were killed in the attack.

The suicide attack adds to the Friday death toll in the Tirah Valley. Members of LeI group ambushed military checkpoints there in the early morning, leaving 33 dead.

The latest reports indicate that at least 10 soldiers and 23 militants were killed in the incident.

Militants from Lashkar-e-Islam group attacked our outpost and a gunfight erupted, Khyber administrator Mutahir Zeb Khan told Agence France Presse (AFP).

 At least 10 soldiers embraced martyrdom and three others were wounded, a senior military official also told AFP.

At least 23 militants were killed in the fighting which lasted nearly six hours. All law enforcement agencies' posts in the area were intact and the situation was stable, he continued.

The numbers of those killed differ depending on the source, and have not been independently verified for either incident.

Pakistan's army launched a major campaign several years ago to assert control of the border area, where militant groups find safe haven due to its remote location and the government's inability to establish law and order. Militant groups maintain a strong presence despite the 100,000 troops in the region.

Khyber was once a crucial land route to Afghanistan for NATO forces. Yet Pakistan closed it to NATO following a skirmish between NATO and Pakistani forces that left 24 Pakistani soldiers dead. The area is also an important trade route between Afghanistan and Pakistan.

About 20,000 residents have been displaced in recent months after the army intensified its operations in the region.