(CNN) -- Philippine President Benigno Aquino III declared a state of national calamity as the death toll from Typhoon Bopha, the strongest tropical cyclone to hit the Philippines this year, continued to climb.
At least 540 people have died since the typhoon, known locally as "Pablo," hit Tuesday, the Philippines' emergency management agency said Sunday, while nearly 1,100 are reported injured.
Another 827 people have been reported missing, the National Disaster Risk Reduction and Management Council said.
Aquino's declaration of a state of national calamity will allow local governments to access funds for rescue, relief and rehabilitation efforts in their areas, the official Philippine News Agency said.
The declaration will also help national and international aid reach stricken communities, and will mean price controls are imposed on basic goods, the news agency said.
The U.S. Defense Department said it will assist in relief operations, with an emphasis on reaching communities isolated by infrastructure damage.
Among the worst hit areas was the poor, remote Compostela Valley region on the southern Philippine island of Mindanao, where thousands of flimsy houses have been ripped to shreds.
Scores of people died, many them swept away in flash floods that roared down from the hills.
The epicenter of the devastation appears to have been in New Bataan, a town about 20 kilometers (12 miles) southeast of Montevista and close to the steep mountains.
The flash floods hit it head-on Tuesday, washing away families huddled in their homes and soldiers stationed in a compound in the town.
On Friday, Aquino visited the area, as well as Boston, in Davao Oriental, to see the destruction for himself, the PNA said.
He ordered government agencies to speed up relief efforts and the reconstruction of roads and bridges to help much needed supplies and assistance reach the affected areas.
"I have Mindanao on my mind, especially the number of people missing. I am hoping these people will be found safe and sound," Aquino was quoted as saying ahead of the visit.
The president said he was saddened that some families would spend the holidays in evacuation centers because their homes were destroyed.
Large parts of New Bataan and many of the people who lived there are now buried under mud, fallen trees and rubble, said Arnaldo Arcadio, an emergency response program manager for Catholic Relief Services, a humanitarian group.
"The mood is really gloomy," he said Thursday after visiting the town, where 90% to 95% of the houses are believed to have been destroyed or damaged.
Residents who evaded death now lack food, shelter and, most of all, drinking water, since the nearest source is 5 kilometers away, according to Arcadio.
"They are just trying to survive," he said, noting that the knee-deep mud in many places made it difficult to get around, with several areas of the town completely inaccessible.
Bopha raked across Mindanao and several other Philippine islands before moving off into the South China Sea.
The powerful typhoon brought savage winds that uprooted entire banana plantations in low-lying areas, and relentless rain that unleashed torrents of rocks and mud down the mountainsides where shanty-dwelling miners dig for gold.
Preemptive action was taken in northern Mindanao, expected to feel the full force of the typhoon, but in the end it was the less prepared communities of Compostela Valley and Davao Oriental, further south and east, that bore the brunt. Many people either didn't hear warnings or didn't heed them, expecting the typhoon to hit further north, as usually happens.
The disaster has left about a quarter of a million homeless and affected more than 5 million people overall, the National Disaster Risk Reduction and Management Council said Friday.
CNN's Laura Smith-Spark, Joseph Netto, Liz Neisloss and Jethro Mullen contributed to this report.