Typhoon Koppu has weakened after battering the northern Philippines on Sunday.
The slow-moving typhoon killed at least two people and forced more than 16,000 people from their homes.
Dozens of villages are flooded and troops have been deployed to help residents trapped on rooftops.
Despite the weakening of the typhoon, officials warn that the danger of landslides and flooding has not passed.
Typhoon Koppu made landfall near the town of Casiguran on the island of Luzon on Sunday morning, bringing winds of close to 200km/h (124mph) and cutting power to vast areas.
By dawn on Monday wind speeds were down to 150 km/h (93 mph) in the northern town of Santiago, the state weather service said.
But rising floodwaters are preventing even military vehicles reaching many of the worst-hit villages and rescuers report a shortage of boats.
While the Philippines is no stranger to typhoons, the slow-moving nature of Koppu, called Lando by local weather authorities, means heavy rain will fall for longer than usual, bringing greater risk of flooding and landslides.
It is not forecast to leave the north of the main island of Luzon until Wednesday, after which it will head towards Taiwan.
Justin Morgan, Philippines country director for the charity Oxfam, said the impact of the typhoon was becoming clearer.
"We are seeing damage as a result of wind, particularly to important infrastructure like hospitals, but also in terms of the flooding. The flooding has been a metre deep in some locations."
He stressed that while evacuations for those still in the path of the typhoon was the most urgent task, "we can expect that there will be needs in terms of helping people recover their livelihoods as we know that many of the farmers would have lost their crops."
Why is Koppu slow-moving? Chris Fawkes, BBC Weather Centre, explains:There are two typhoons in the west Pacific at the moment - Typhoon Champi sits just to the east of Koppu.
The complex interaction between these two typhoons and the warm air within these storms helps to build a ridge of high pressure over Taiwan this weekend. It is this ridge that effectively traps typhoon Koppu over the Philippines for a number of days rather than it being able to turn away from the Philippines and out of harm's way to the South China Sea
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