DERA ISMAIL KHAN- The militant group that said it was behind Wednesday's massacre of 45 commuters in Karachi is a dangerous outfit with ties to Pakistan's Taliban, but intelligence sources voiced doubts about claims it had received financial support from Islamic State.

Jundullah, a Sunni Muslim organization that targets Pakistan's minority Shi'ite community, pledged allegiance to Islamic State (IS) last year, one of several smaller jihadist groups in South Asia to do so.

It also said in November that its leaders held a meeting with a three-man delegation representing IS.

Ahmed Marwat, who purports to be a Jundullah commander and spokesman, went further this week, telling the group had received money from IS, the ultra-violent jihadist movement that controls areas of Iraq and Syria.

"ISIS brothers are supporting us a lot financially," he said by telephone from an undisclosed location. Marwat added that Jundullah planned more attacks and its aim was to impose sharia, or Islamic law, in Pakistan.

Leaflets left at the scene of Wednesday's gun attack on a bus carrying Shi'ite Ismailis referred to Islamic State and expressed hatred for Shi'ite Muslims.

Later a Twitter account from militants identifying themselves as Islamic State claimed responsibility. It was not possible to verify their claims.

Any proof that IS has direct links with some of the thousands of Islamist militants in Pakistan and Afghanistan would spark alarm in a region where the Taliban, al Qaeda and myriad other groups carry out atrocities almost daily.

But, while analysts believe militants draw inspiration from the IS' military successes and grisly propaganda, intelligence officials in Pakistan said they doubted IS had financial ties with Jundullah.

"We have no evidence of any money trail of this kind," said one military intelligence official, when asked about Marwat's claims. "This is deep exaggeration."

Another official said Jundullah financed itself through bank robberies, extortion and kidnapping.