Severe flooding across Myanmar hampered rescue efforts on Saturday as thousands sheltered at monasteries after rising waters triggered by torrential rains killed at least 27 people, officials said.

Heavy monsoon rain has battered vast swathes of the country in recent weeks, decimating tens of thousands of homes and fields and leaving people stranded in remote villages 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 who did not want to be named, 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 and on Friday President Thein Sein declared them "national disaster affected regions", according to the state-backed Global New Light of Myanmar (GNLM) newspaper.

These regions "have seen huge destruction and face difficulty returning to normal conditions", GNLM reported, citing the government announcement.

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 told AFP, adding that rescue camps have been opened at monasteries.

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, told AFP.

Myanmar is struck by annual monsoon rains that are a lifeline for farmers but can also prove deadly, with landslides and flash floods a common occurrence.

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