Nine people have died and hundreds have fallen ill with diarrhoea in areas of the Philippines wrecked by a typhoon last month, with aid officials warning of a health crisis as millions struggle to secure clean water and food.

Three weeks after Typhoon Rai struck southern and central islands, destroying thousands of homes and killing more than 400 people, relief work continues to deliver supplies to stranded residents left homeless by the storm.

Nine people have died from dehydration caused by diarrhoea in the impoverished Dinagat Islands and the neighbouring resort island of Siargao, known for its surf spots, the Philippine health department's regional office said.

A total of 895 cases were recorded there since the typhoon struck, mostly among those left homeless, as the government and aid agencies rush to build emergency water treatment facilities, department spokesman Ernesto Pareja told AFP.

"It's hard to say it's under control. The water supply remains irregular. Their food needs have not been addressed," Pareja said.

The International Federation of Red Cross and Red Crescent Societies (IFRC) warned of a "mounting health crisis" in typhoon-hit areas as it scaled up its disaster response.

"It is extremely concerning that people have been getting very sick and even dying in areas smashed by this typhoon," IFRC Head of Philippine Delegation Alberto Bocanegra said in a statement Thursday.

The typhoon left "millions without access to clean drinking water, hospitals and health facilities", Bocanegra added.

A total of 402 people died from the typhoon, according to the Philippine civil defence office's latest tally, with more than 1,200 injured and 78 still missing and at least 370,000 still in evacuation centres.