The toll from flash floods and landslides in Myanmar caused by days of torrential rain is likely to rise, the UN warned Sunday, as monsoon downpours heaped misery on thousands across the region.

At least 27 people have been killed and more than 150,000 affected by flooding in Myanmar in recent days, with the government declaring the four worst-hit areas in central and western Myanmar "national disaster-affected regions".

Scores have also perished in India, Nepal, Pakistan and Vietnam following floods and landslides triggered by heavy seasonal rains.

Rescue work in Myanmar has been hampered by continued downpours and the inaccessibility of many of the remote regions battered by the deluges.

In Kalay, one of the worst-hit towns in the country's northwest Sagaing region, floodwaters on Sunday had risen as high as the roofs of houses and above the tops of coconut trees, an AFP photographer at the scene said.

Vast tracts of farmland had been swallowed up by the flooding, turning a normally fertile flat valley into an expansive lake.

An official at Myanmar's Relief and Resettlement Department, who asked not to be named, told AFP that at least 166,000 people have now been affected by the floods.

But the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) said the real figure was likely to be "significantly higher" because many areas "have still not been reached or reported on by assessment teams".

OCHA said the official death toll of 27 was also likely an underestimate.

"As further information becomes available, this figure is also expected to increase," the statement warned.

- Regional deluge -

Seasonal monsoon rains have also brought death and destruction to other Asian nations.

The death toll in India had passed 100, officials said late Sunday.

About 20 people were feared dead after a hill collapsed onto a village in the northeastern state of Manipur on Saturday following incessant rain, a local magistrate said.

Rescuers were Sunday clawing through mud and debris searching for survivors and victims of the accident in the remote village in Chandel district, which borders Myanmar.

"So far we have reports of 20 people killed when a hillock caved and trapped the villagers," magistrate Memi Mary told AFP by telephone from Chandel town.

The Press Trust of India reported a further three people had drowned in the floodwaters in Manipur over the past few days.

In the worst-hit western state of Gujarat the death toll stood at 53.

In West Bengal, some 42 people have been killed in the last week from flooding, while some 250,000 homes have been destroyed, the state's minister for disaster management, Javed Ahmad Khan, said.

Meanwhile 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.

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

"In one second, mud and rock smashed into my house. We were lucky to escape with our daughter," To Thi Huyen, a 37-year-old primary school teacher, told AFP.

Inundations have also hit Pakistan with 109 killed and almost 700,000 affected by floods in the last two weeks, while 36 people have perished in landslides in Nepal.

- Homes and bridges collapsed -

Two of the worst-hit areas in Myanmar are the remote, impoverished western states of Chin and Rakhine.

The Myanmar Red Cross Society said 300 homes in Rakhine had been destroyed or damaged, with around 1,500 people evacuated to shelters.

"The figures are expected to increase in the coming days as Red Cross assessment teams access remote areas of Rakhine affected by the flooding," the agency's head Maung Maung Khin said in a statement Sunday.

Rakhine already hosts some 140,000 displaced people, mainly Rohingya Muslims, who live in exposed makeshift coastal camps following deadly 2012 unrest between the minority group and Buddhists.

Rescue workers have been mobilised across the country but the sheer extent of the flooding is testing the government's limited capacity to cope with disasters, officials admit.

In Bago region, three hours north of Myanmar's commercial hub Yangon, floodwaters had forced more than a thousand people to take shelter in a monastery.

"There's just too much rain this year and the dams had to let the water out," construction worker Hla Wai, whose house was partially underwater, told AFP at the monastery.

Myanmar's annual monsoon is a lifeline for farmers but the rains and frequent powerful cyclones can also prove deadly, with landslides and flash floods a common occurrence.