Visiting Abu Bakr Bashir at Jakarta's Salemba prison on Thursday, Din Shams al-Din said the US embassy was trying to have the 65-year-old's sentence lengthened.
"I know the US embassy via its ambassador in Jakarta has asked some Islamic figures to help keep Abu Bakar Bashir in jail. This is clearly intervention."
Bashir is due to be freed on 30 April after serving time for immigration offences.
But last week police named him a suspect in a fresh terror investigation, which could allow authorities to keep the 65-year-old preacher detained.
US ambassador to Jakarta, Ralph Boyce, has denied trying to intervene in Indonesia's legal system, although Washington has made clear it wants Bashir to remain behind bars.
In the latest investigation, police are focusing on possible links to the 2002 Bali bombings and the preacher's alleged leadership of JI.
Bashir denies links to the Islamist movement blamed for the October 2002 Bali bomb attacks that killed 202 people.
Previous charges against Bashir of leading Jemaah Islamiah and treason were either unproven, dismissed or overturned.