"A lot of roofs from well-constructed buildings have been blown off," Tony Craig, regional emergency co-ordinator for the World Food Programme (WFP), told the Reuters news agency.
"That would lead you to believe that less well-constructed buildings will have taken a really big whack."
An official at Yangon International Airport said all incoming flights had been diverted to the second city of Mandalay, in the middle of the southeast Asian nation, and all departures from Yangon had been cancelled.
An official at Thai Airways in Bangkok said the airline planned to resume flights on Sunday.
There were no immediate reports of casualties but an official in the capital Naypyidaw told the AFP news agency that some fishing boats were missing.
The Associated Press news agency reported an official from the country's meteorology department, who refused to be named because he was not authorised to speak to the press, as saying: "The cyclone wreaked havoc in Yangon."
"The damage will be extensive as the cyclone passed through many densely populated areas," the official said.
States of emergency
The Federation of Trade Unions, Burma said from Thailand that the military government had declared states of emergency in five affected provinces, most of them in the low-lying floodplains of the Irrawaddy delta.
Cyclone Nargis made landfall around the mouth of the Irrawaddy river, about 220km southwest of Yangon, before hitting the country's economic hub.
Tens of thousands of people were made homeless in the Irrawaddy delta last August after unusually heavy rains triggered floods in the low-lying region.
On Saturday evening it was forecast to move northeast towards Thailand, which warned that flash floods could hit the north, centre and east of the country and said heavy rains were expected until Monday.