One of the strongest storms to hit Myanmar in recent years severely disrupted communications in the impoverished state of Rakhine, a major ethnic armed group and an aid worker said on Monday, making it hard to ascertain the scale of its impact.

Cyclone Mocha barrelled into the western coast of Myanmar from the Bay of Bengal on Sunday afternoon, largely sparing over a million refugees in vulnerable camps in neighbouring Bangladesh, but flooded Rakhine's capital of Sittwe and took down at least one communications tower.

Calls by Reuters reporters to 11 phone numbers the region did not go through and at least another one dozen people said they couldn't get in touch with family members in Sittwe.

Some 400,000 people were evacuated in Myanmar and Bangladesh ahead of Cyclone Mocha making landfall, as authorities and aid agencies scrambled to avoid heavy casualties.

"All communication is still down and people are in trouble because all the roofs are gone,” said Khine Thu Kha, a spokesman for the Arakan Army, which control swathes of Rakhine state.“We are using military devices to communicate with them."

Benjamin Small, a consultant with the United Nations Development Programme, said it was hard to understand the scale of destruction because of ruptured communications in Rakhine.

"But with reported 250 kph (155 mph) winds making Cyclone Mocha one of the strongest in Myanmar on record and the worrying images online, it's not looking good," Small said on his Twitter account.

In 2008, Cyclone Nargis swept across parts of Myanmar with winds of 240 kph (150 mph), killing nearly 140,000 people. Since a junta topped a democratically elected government two years ago, Myanmar has been plunged into chaos and a resistance movement is fighting the military on various fronts following a crackdown on protests.

A junta spokesperson did not immediately answer a telephone call from Reuters seeking comment.

The military has imposed internet shutdowns across parts of the country, including some areas in Rakhine and neighbouring Chin state, which was also in Cyclone Mocha's path.

"There is significant overlap between the shutdown areas and the cyclone's path, which is a major concern, as it's hindering efforts to reach people," said digital rights activist Htaike Htaike Aung.

In remote and hilly Chin, which has previously seen heavy fighting between the junta and the resistance, the areas the storm swept through is under a communications blackout since the coup, the Chin Human Rights Organization said.

"We have not yet been able to establish the extent of the devastation,” said the group's deputy director Salai Za Uk Ling. "The storm itself is a trigger for more problems as heavy rains continue and landslides and flooding tend to follow."