Sambo Dasuki spent four years in prison despite multiple court orders to Nigeria's Department of State Services (DSS) domestic intelligence agency for his release on bail.

Dasuki is accused of siphoning money earmarked for the fight against Islamic militant group Boko Haram and allegedly using it to help bankroll former president Goodluck Jonathan's failed re-election campaign in 2015.

"He was released Tuesday night and has since reunited with his family in Abuja," a senior DSS officer told AFP.

"Remember Dasuki was granted bail by the court," the source said, adding: "He can travel out of the country to seek medical attention but with the permission of the court since his case will have to continue."

Nigeria's intelligence agency has come under attack from critics for ignoring court orders to release certain prominent prisoners.

On Tuesday, the DSS freed a prominent journalist and opposition activist Omoyele Sowore who had been held since August despite court orders demanding his release.

Sowore -- a publisher of online news site Sahara Reporters -- still faces trial for treason and other crimes but was freed on bail under orders from the attorney general.