Philippines troops thwarted an original plan drawn up by the Islamist militants now holed up in Marawi City to "spread terror" in a rampage of violence that would have given them full control of the southern town, the military said on Wednesday, June 7, according to Reuters.

"There was indeed a bigger plan and it was supposed to wreak more havoc," military spokesman Restituto Padilla told a news conference as aerial bombing resumed against fighters who have sworn allegiance to the ultra-radical Islamic State.

Philippine senators said members of the upper house were last week shown a video of the militants, led by a group known as Maute, plotting a far more sophisticated siege of Marawi City than the attack they launched there on May 23.

"It was clear that these terrorists, the Maute group, their end goal is to make Marawi ... independent, or to separate from the republic," Senator JV Ejercito told Reuters.

"With a plan like this, this is already rebellion and a threat to national security, so declaration of martial law is justified," he said, referring to the martial law declared by President Rodrigo Duterte across the southern island of Mindanao when the siege began.

The Associated Press (AP) first reported on the seized video footage, which showed Isnilon Hapilon - proclaimed last year by Islamic State as its "emir" of Southeast Asia - and others planning to take hostages from a school, seal off roads and capture a highway.

Armed forces Chief of Staff General Eduaro Año told the AP that the images showed an intention to dismember "a portion of the Philippine territory by occupying the whole of Marawi City and establishing their own Islamic state or government".