Rescuers raced on Monday to help tens of thousands of people in Myanmar who were stranded by rooftop-high floods, as the UN warned that swollen rivers threaten more areas and large swathes of Asia that have been hit by deadly monsoon rains.

Authorities in Myanmar said the death toll from flash floods and landslides caused by weeks of unrelenting rain rose to 46, with some 200,000 affected and villagers in remote areas forced to use canoes and makeshift rafts.

Hundreds have died in recent days in India, Nepal, Pakistan and Vietnam in floods and landslides triggered by a belt of heavy seasonal rains.

Access to many towns in remote northern and western Myanmar has been severed, and relief workers fear it could be days before the true extent of the disaster emerges.

"Logistics are extremely difficult. Assessment teams are having a hard time reaching affected areas," said Pierre Peron, spokesman for the United Nations Office for the Coordination of Humanitarian Affairs.

The UN was "very concerned" by the situation, he said, adding that while flooding had begun to recede in some places, rivers were bursting their banks and inundating new areas.

Relentless downpours were exacerbated last week by Cyclone Komen, which drove fierce winds and heavy rain across the western part of the country.

Residents near Kalay, a town in the impoverished northwestern Sagaing region that remains virtually encircled by deep water, described how their homes were swallowed by the deluge.

"There was no warning. ... We thought it was normal" seasonal flooding, said Aye Myat Su, 30, at a monastery being used as a temporary shelter in Kalay.

"But within a few hours, the whole house was underwater. My husband had to get onto the roof as there was no way out."

An AFP photographer in the area said floodwaters remained high on Monday, with many people making their way to safety in rafts cobbled together from old tires, salvaged wood and large plastic bottles.

Myanmar's annual monsoon is a lifeline for farmers, but the rains and frequent powerful cyclones can also prove deadly.

Poor infrastructure and a limited government search and rescue capability have hampered relief efforts across the nation, with roads, phone lines and electricity knocked out by the rising water.

"We are speeding up assistance and relief work," an official at the Relief and Resettlement Department said, asking not to be named.

Authorities have declared the four worst-hit areas in central and western Myanmar "national disaster-affected regions". Landslides in Chin state - south of Sagaing - have destroyed 700 homes in the state capital, Hakha, according to the state-run Global New Light of Myanmar.

President Thein Sein has promised the government will do its "utmost" to provide relief.

Residents make a raft to travel through floodwaters on Monday in Kalay, in upper Myanmar's Sagaing region. Relentless monsoon rains have triggered flash floods and landslides, destroying thousands of houses, farmland, bridges and roads. Fast-flowing waters are hampering relief efforts. Ye Aung Thu / AFP

(China Daily 08/04/2015 page11)