President Rodrigo Duterte's constitutional mandate allows him to impose martial law across the entire country, with the rebellion brewing in the country's south providing justification for the decision, the Supreme Court ruled this week.

In an 82-page ruling made public Thursday, two days after it was handed down, the Supreme Court said the Constitution grants Duterte "the discretion to determine the territorial coverage of martial law," whether it be the entire country or just part of it.

The decision, signed by 11 of 15 top court justices, said the Constitution does not confine the declaration of martial law to the particular location where the unrest is actually taking place and that the president can impose it wherever he believes there is the possibility of a spill-over in hostilities.

It only noted that a time limit of 60 days is in place for a declaration of martial law.

• READ: The Supreme Court decision affirming Mindanao martial law

Duterte announced martial law would be adopted in Mindanao on May 23 after heavily armed Islamic State-linked fighters occupied areas within the predominantly Muslim city of Marawi.

According to the same decision handed down by the high court, in declaring martial law the president has "reasonable basis to believe that Marawi is only the staging point of the rebellion" and that the militants' ultimate goal could be to take control of not just the city but the whole Mindanao region.

"Considering the widespread atrocities in Mindanao and the linkages among rebel groups, the armed uprising that was initially staged in Marawi cannot be justified as confined only to Marawi," the ruling said.

It also said that the argument in petitions to the Supreme Court that the incident is just an instance of terrorism rather than a rebellion does not hold, as the former does not negate the latter.

"Rebellion may be subsumed under the crime of terrorism, which has a broader scope covering a wide range of predicated crimes," the ruling said.

Journalists were briefed by the court's spokesman on Tuesday about the ruling, which upheld the constitutionality of Duterte's decision to impose martial law in Mindanao. But the full text of the ruling was only made public on Thursday.

Intense fighting between government forces and militants has seen the government carry out airstrikes amid heavy fighting that has dragged on for more than a month in the city's streets.

The death toll has reached more than 450 people, according to authorities, including some 336 militants, 84 government troops and 39 civilians.

• IN BRIEF: SC decision upholding Duterte's martial law in Mindanao