The chief of Pakistan's anti-graft watchdog has resigned, apparently due to pressure from the Supreme Court to arrest Prime Minister Raja Pervez Ashraf in connection with two corruption cases, sources said today.

"National Accountability Bureau Chairman Fasih Bokhari has called it a day and submitted his resignation to President Asif Ali Zardari," a source privy to the development told PTI.

"A series of events that landed NAB in hot waters is the reason behind Bokhari's decision to resign," the source said.

During a recent hearing in in the Supreme Court, Bokhari had a heated exchange with Chief Justice Iftikhar Chaudhry for not showing respect to him.

Bokhari, a former naval chief, told the top judge in a firm tone to call him "Admiral Bokhari instead of Mr Bokhari", surprising people present in the courtroom.

The NAB chief has also been at the centre of a controversy over the mysterious death of investigator Kamran Faisal, who was probing graft charges against the premier.

An interim autopsy report concluded Faisal had committed suicide but his relatives and colleagues have disputed the findings, saying the investigator was being pressured by Bokhari to change a report that he had submitted against the premier.

The source said Bokhari was facing intense pressure from the Supreme Court to file a case against Prime Minister Ashraf, Interior Minister Rehman Malik and senior Pakistan Peoples Party leader Jehangir Badr in connection with alleged corruption.

The apex court has sought the filing of a case against the premier for appointing Sadiq, who has been accused of causing losses of Rs 83 billion to the national exchequer.

Malik and Badr have been accused of obstructing investigations and facilitating Sadiq's escape from Pakistan. Sadiq is currently believed to be in Dubai.

Earlier, the Supreme Court had directed NAB to arrest the premier and 16 other suspects for alleged involvement in corruption in setting up rental power projects.

However, Bokhari had informed the court that he did not have enough evidence to make any arrests.

The mysterious death of Kamran Faisal has compounded this case. A judicial commission and a Supreme Court bench are conducting parallel investigations into Faisal's death.