YANGON : Myanmar residents awoke Sunday to devastation after tropical cyclone Nargis tore through city streets, battered buildings, sank boats and caused unknown casualties, officials said.

The cyclone has also ripped down power and phone lines, cutting off the military-run nation just a week before a crucial referendum on its new constitution, the first polling in Myanmar since general elections in 1990.

The main city of Yangon was hard hit, with traffic lights, billboards and street lamps littering the streets after being blown over by the strong winds that swept through on Saturday, an AFP reporter on the scene said.

Trees have been uprooted, crushing buildings and cars.

Five central and southern regions -- Yangon, Ayeyawaddy, Bago, Mon and Karen states -- have all been declared disaster areas, an official at the information ministry told AFP.

"So far we know there were casualties (deaths), but we cannot release the details yet as we are still collecting information," he said.

"We also deployed military units for rescue and rehabilitation projects. Now the military and police have started to clean the city," he said.

On the streets of Yangon, rumours swirled that two women had been crushed to death by trees, but there was no official confirmation.

Roofs of houses have been torn away, while only a few taxis and buses -- which tripled their prices -- braved the debris-clogged streets on Sunday.

The information ministry official said that seven empty boats had sunk in the country's main port, while Yangon's international airport was closed until further notice with flights diverted to the city of Mandalay.

"We are trying to get back to the normal situation as soon as possible," he said, adding that the prime minister and other officials had left the isolated administrative capital Naypyidaw and headed to Yangon for rescue efforts.

Electricity supplies and telecommunications in Yangon have been cut since late Friday night as the storm bore down from the Bay of Bengal, packing winds of 190-240 kilometres (120-150 miles) per hour.

Nargis made landfall around the mouth of the Ayeyawaddy (Irrawaddy) river, about 220 kilometres (137 miles) southwest of Yangon, before hitting the country's economic hub of Yangon.

It was not immediately known whether damage from the storm would affect the referendum next Saturday on a new constitution which the ruling government says will pave the way for democratic elections in 2010.

Thailand's meteorological department on Sunday downgraded Nargis to a depression, but warned of flash floods and heavy rains.

- AFP/vm