By Asif Shahzad

ISLAMABAD Pakistani authorities have opened a criminal investigation against jailed former prime minister Imran Khan on charges of leaking state secrets, after naming him and three aides in a fresh case, a top security source said on Monday.

The matter, currently under investigation, pertains to a classified cable sent to Islamabad by Pakistan's ambassador in Washington early last year, which Khan is alleged to have made public.

The 70-year-old former cricketer has accused that the cable was part of a U.S. conspiracy to push the Pakistani military to oust him in a parliamentary vote of no confidence in 2022 because he had visited Moscow ahead of Russia's attack on Ukraine.

Both Washington and the military deny this.

Khan is currently serving a three-year sentence in a graft case and has been barred from politics for five years.

"Our investigation is collecting evidence to stand a case in a court to indict Imran Khan on charges of leaking official secrets," a security source, who is directly responsible for the investigation, told Reuters.

Khan's party's information secretary Rauf Hasan did not respond to a request for comment.

His close aide Zulfi Bukhari, however, said such a charge against Khan would be unconstitutional after the law became controversial with an assertion by President Arif Alvi that he never signed recent amendments to the legislation, which was mandatory.

Khan has formally been arrested in connection with the charges, which the Federal Investigation Agency (FIA) is probing, the source said.

One of the three aides named in the case, former foreign minister Shah Mehmood Qureshi, was arrested on Saturday and sent to the FIA's custody by a court on Monday for four days, his lawyer Intazar Panjutha said.

A copy of the FIA case seen by Reuters said Khan and his aides disclosed the classified documents to unauthorised persons and were "twisting the facts to achieve their ulterior motives and personal gains".

Under Pakistan's Official Secrets Act, a guilty sentence could range from two to 14 years in prison or even death, lawyers say.

Khan used the secret document for his "vested interest at the cost of national security", the case says, adding that the former premier also retained illegally a copy of the classified cable.

Khan has been at the centre stage of politics since his removal, with a countrywide protest movement and going through more than 100 cases against him.

(Reporting by Asif Shahzad; Editing by Devika Syamnath)