MANILA - Philippine President Rodrigo Duterte says he cannot guarantee that other parts of the country are safe from attacks by the Islamic State organisation, a group he likened to German dictator Adolf Hitler.

Hitler, said Duterte, killed millions of Jews "for nothing."

Duterte said he could not even tell when a siege by hundreds of militants allied with the terrorist group in Marawi City, 800km south of Manila, would end, nor what to expect in the coming days.

"It if ends tomorrow, I'll be the most happiest man in the country," Duterte told reporters after receiving millions of dollars worth of rifles and ammunition donated by China.

When asked if he could guarantee that a similar attack in other parts of the country would not happen, the 72-year-old leader said, "I cannot give that assurance."

"It's bereft of ideology," he said, referring to the Islamic State. "What they know is just to kill and destroy ... It's a mass insanity."

"Every generation has that kind of [phenomenon]. During our parents' time, Hitler was a madman, and yet he was able to contaminate a lot of people with his ideas about killing Jews.

"Killing by the millions for nothing. That's just like ISIS."

Troops have been battling the militants for more than a month in Marawi City, where rescuers on Wednesday recovered 17 decomposing bodies of civilians believed to have been killed by the terrorists.

The retrieval brought to 44 the number of civilians allegedly killed by the militants during the five-week conflict, said Brigadier General Restituto Padilla, a military spokesman.

Padilla said the number of civilians killed by the militants allied with Islamic State could still increase as reports of other executions are validated.