Myanmar formally appealed Tuesday for international help in tackling massive floods with rescuers struggling to reach isolated regions, as rain-battered Asian nations counted the rising cost of this year's monsoon.

Flash floods and landslides in Myanmar have claimed at least 46 lives and affected some 215,000 people, swallowing huge tracts of land in what the United Nations has described as a "major natural disaster".

Tens of thousands of people remain cut off as officials warn that swollen rivers are now threatening to inundate low-lying southern areas of the country.

"Myanmar authorities have today formally requested international assistance," the United Nations Office for the Coordination of Humanitarian Affairs said in a statement, adding that the UN and other aid agencies have however been part of the response since late July.

Military helicopters and commercial airlines have helped deliver international food aid as Myanmar shows a willingness to allow foreign assistance that was unthinkable under the paranoid former junta.

In 2008 the military regime was widely criticised for refusing foreign aid for weeks after Cyclone Nargis wrought devastation across the Irrawaddy Delta, leaving nearly 140,000 people dead or missing.

Now, with a fast-growing troop of social media users, the quasi-civilian government has already come under attack for its flood response, accused of underplaying the scale of the disaster.

Government spokesman Ye Htut told AFP that Myanmar had sent an official list of requirements to the UN on Tuesday as the scale of the crisis was overwhelming local recovery efforts.

"We responded immediately. But in some areas there are no resources, such as boats to rescue people," he said, adding that parts of the northern Sagaing region were seeing their "worst natural disaster in 100 years".

Parliamentary spokesman Shwe Mann, a former general, issued a statement Tuesday calling for urgent international assistance "in order to save many lives".

Relentless seasonal downpours have lashed much of South and Southeast Asia in recent weeks.

The annual monsoon is a lifeline for farmers across Asia but heavy rains and powerful cyclones can also prove deadly.

By Tuesday the death toll in India from days of rain had risen to 180, the majority in West Bengal after receding waters uncovered more bodies.

Around 1.2 million people have been forced from their homes after rivers burst their banks in the wake of Cyclone Komen -- which barrelled through the Bay of Bengal late last week.

Pakistan has seen 118 people die so far, with 810,000 affected, as poorly built mud homes collapsed under heavy rains.

Scores have also died in Vietnam and Nepal.

- 'Major natural disaster' -

Myanmar has designated four states and regions -- Rakhine, Chin, Sagaing and Magway -- as "natural disaster" areas and warned that flooding is spreading southwards.

Roads are damaged or inundated, phone lines are down and electricity has been cut to large areas, raising fears that tens of thousands of people -- many of whom already live in abject poverty -- are in dire need of help.

Soldiers loaded aid onto helicopters in the Rakhine state capital Sittwe early Tuesday, an AFP reporter at the scene said, as rescuers battled to reach remote communities in the rugged and impoverished state that remain virtually cut off.

Flooding has started to recede in the region around the city, but thousands of homes and farms have been destroyed.

A Sittwe police officer requesting anonymity told AFP that by Sunday 37 people had died from floods in the state, indicating the final national toll could surge, while seven were missing.

Floodwaters reached rooftop level in many parts of Myanmar, forcing people into canoes or makeshift rafts to reach safety.

Warning the toll was likely to rise, UN Acting Resident and Humanitarian Coordinator Eamonn Murphy said relief resources would soon be stretched across the country.

"It is quite clear that we have a major natural disaster," he said, adding that Myanmar's government had been "much more prepared" this year compared to previous disasters.

Ye Htut however said the government was struggling to deliver aid to the worst-hit areas such as mountainous Chin state bordering India.