A Pakistani court has extended the jail custody of former prime minister Imran Khan for 14 days to investigate him on charges of leaking state secrets, his lawyer says, a day after another court suspended his prison sentence for graft.

The special court held the proceedings on Wednesday at Attock Jail, where Khan began the three-year prison term on August 5 after being found guilty of unlawfully selling state gifts.

A high court suspended that sentence on Tuesday, ordering Khan to be released on bail, but he was barred from leaving as he was still under remand in the official secrets case.

Speaking to journalists outside the jail after the judge extended Khan's remand until September 13, Khan's lawyer Naeem Panjutha said an application for bail had been submitted and would be heard on Saturday.

"We have also filed a plea for an open court trial," Panjutha said, anticipating the prosecution could seek a closed-door trial, without media or public present.

Multiple cases have been lodged against the 70-year-old former national cricket captain since he lost the premiership in a parliamentary confidence vote in April last year.

Khan denies any wrongdoing and says the accusations against him are politically motivated.

Khan's supporters believe their leader is being punished for having the temerity to challenge the military's dominant influence in Pakistan's politics, and that the courts are being used to keep him out of a national election that is due later this year but could be delayed until early 2024.

While the sentence in the graft case has been suspended, the conviction still stands, giving the Election Commission no reason to remove the five-year ban on Khan contesting elections.

Khan has been charged under the Official Secrets Act for making public the contents of a confidential cable sent by Pakistan's ambassador to the United States and using it for political gains, according to a Federal Investigation Agency (FIA) case report seen by Reuters.

His top aide, former Foreign Minister Shah Mahmood Qureshi, has already been arrested and questioned in the case.

Khan alleges the cable showed the US was pressing Pakistan's military to topple his government by warning of consequences for Pakistan if the confidence vote failed to remove him.

Washington and Pakistan's military have denied that.

But conspiracy theories abound in Pakistan - a country where no elected prime minister has completed their term.

The US State Department issued a statement saying senior US diplomat Victoria Nuland spoke on Tuesday with Pakistani Foreign Minister Jalil Abbas Jilani and discussed the importance of "timely, free and fair elections", as well as Pakistan's economic stability and the need for continued engagement with the International Monetary Fund (IMF).

Khan faces dozens of cases, including charges of corruption, abetment to murder, treason and orchestrating violent protests that followed his initial arrest in May.

He denies all the charges.