Rescuers raced Monday to help tens of thousands stranded by rooftop-high floods in Myanmar, as the UN warned that swollen rivers threaten more areas and large swathes of Asia were 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 also perished in recent days in India, Nepal, Pakistan and Vietnam following 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 last week exacerbated 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)," Aye Myat Su, 30, told AFP from 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 tyres, salvaged wood and large plastic bottles.

- Toll rises -

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

Poor infrastructure and 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 told AFP, asking not to be named, adding 46 people had died in the floods as of Sunday, with some 200,000 affected.

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, but said parts of Chin had been cut off from surrounding areas, the report added.

The health ministry says it is distributing medical supplies across the country including chlorine tablets, although it was unclear how they will reach many of the afflicted zones, with boats and helicopters in short supply.

Rains have also battered the western state of Rakhine which already hosts about 140,000 displaced people, mainly Rohingya Muslims, who live in exposed coastal camps following deadly 2012 unrest between the minority group and Buddhists.

In May 2008 Cyclone Nargis devastated Myanmar's Irrawaddy Delta, killing about 140,000 people.

Elsewhere in Asia, flooding left more than 100 people dead in India, officials there said late on Sunday, while a similar number have been killed in Pakistan over the past two weeks.

A spokesman for Pakistan's National Disaster Management Agency told AFP that 116 people had died and more than 850,000 people had been affected around the country by this year's monsoon floods.

In Vietnam rescuers were battling toxic mudslides from flood-hit coal mines in the northern province of Quang Ninh, home to the UNESCO-listed Halong Bay tourist site.

Twenty three people have been killed in recent flooding in Vietnam, including two families swallowed up by the toxic mud.