The US military claims to have "successfully" disrupted the online propaganda efforts of the Islamic State in a hacking operation dating back at least to 2016, according to declassified national security documents released Tuesday.

The heavily redacted, previously top secret documents said the US Cyber Command "successfully contested ISIS in the information domain" and limited its online efforts on radicalization and recruitment "by imposing time and resource costs" on the jihadist group.

The documents released by the National Security Archive at George Washington University offer the most detailed look at "Operation Glowing Symphony," the first offensive hacking operation acknowledged by the Pentagon.

The assessment pointed to a "significant reduction" in the online campaign waged by ISIS, but added that the Cyber Command efforts were slowed by a "lengthy and difficult" process for approving its operations.

It said that, given the expectation of "more frequent and widely scoped cyber operations," better procedures should be in place to "help expedite the request and approval process."

Officials have previously acknowledged the use of offensive cyber weapons as part of the US arsenal, but the newly released documents offer the most detailed assessment of the moves against ISIS by a joint task force created in 2016 by president Barack Obama.

According to a statement from the university archive, "Glowing Symphony" was initially approved for a 30-day window in late 2016 but a July 2017 administrative message extended the operation, and it was unclear if it is continuing.

The documents released under a Freedom of Information Act request "reveal the unprecedented complexity of the operation, resulting challenges in coordination and deconfliction, and assessments of effectiveness," the GWU archive said.