At least 27 people have been killed after torrential rains caused severe flooding across Myanmar, with floodwaters hampering efforts to rescue thousands stranded in shelters, government officials have said.

In neighbouring India's Manipur state, which borders western Myanmar's Chin state, at least 20 people were killed in a landslide triggered by incessant rain in Chandel district, Indian reports say.

Heavy monsoon rain has battered vast swathes of the country in recent weeks and on Friday the government declared the four worst-hit areas in central and western Myanmar as "national disaster affected regions", indicating the severity of the damage.

Around 150,000 homes and fields have been decimated, leaving people stranded in remote villages and destroying their livelihoods in a disaster testing the government's limited relief operations.

"Most of the country is flooded now," said a director at the social welfare ministry, explaining that all but one of Myanmar's 14 provinces and regions were affected by the flash floods, rising waters and landslides caused by the downpour.

Rescue efforts by Myanmar authorities and local aid groups were under way but they are "struggling to access flood-hit areas", the official said, adding that there was no update to Thursday's death toll of 27 due to disrupted communications.

Western Rakhine and Chin states are among the four worst affected areas.

In the announcement signed by president Thein Sein, it said these areas had seen "huge destruction and face difficulty returning to normal conditions", according to the state-backed Global New Light of Myanmar (GNLM) newspaper.

In Maungdaw township in Rakhine, houses and office buildings have been destroyed, trees uprooted and damaged roads sealed off due to violent storms, a local government official said.

A resident makes his way through floodwaters in Kalay, Sagaing Region, Upper Myanmar. ( AFP/Sai Zaw )

He added that rescue camps have been opened at monasteries while transport in the area was reduced to "almost zero" due to the continuous rainfall.

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

"More than 7,000 people are sheltering in rescue camps at 23 monasteries in Minbyar town. We need drinking water urgently. Our road communication is cut," Khin Zaw Win, a resident in Minbyar, a town in northern Rakhine, said.

A video on the news website Democratic Voice of Burma showed rescuers in the central township of Kawlin evacuating some residents by stretcher while wading through chest high waters.

Myanmar is annually struck by monsoon rains that are a lifeline for farmers, but those along with frequent powerful cyclones can also prove deadly, with landslides and flash floods a common occurrence.

Cyclone Nargis, which devastated Myanmar's Irrawaddy Delta in May 2008, killed about 140,000 people.

This year's floods have so far destroyed at least 30,000 acres of farmland, according to GNLM, and damaged a further 73,000.

A United Nations envoy for Myanmar expressed serious concern over the flooding in a statement, saying assessment teams had been dispatched to the worst-hit areas to identify immediate relief priorities.

A bridge washed away by the floodwaters in India's Manipur state. ( AFP )

AFP