YANGON, Burma — A powerful cyclone killed more than 350 people and destroyed thousands of homes, state-run press said yesterday. Some dissident groups worried that the military junta running Burma would be reluctant to ask for international help.

Tropical Cyclone Nargis hit at a delicate time for the junta, less than a week ahead of a crucial referendum on a new constitution. Should the junta be seen as failing disaster victims, voters who already blame the regime for ruining the economy and squashing democracy could take out their frustrations at the ballot box.

Some in Yangon complained the 400,000-strong military was doing little to help victims after Saturday’s storm.

“Where are all those uniformed people who are always ready to beat civilians?” said a trishaw driver who refused to be identified for fear of retribution. “They should come out in full force and help clean up the areas and restore electricity.”

Burma has been under military rule since 1962. Its government has been widely criticized for human rights abuses and suppression of pro-democracy parties such as the one led by Nobel Peace Prize laureate Aung San Suu Kyi, who has been under house arrest for almost 12 of the past 18 years.

The Forum for Democracy in Burma and other dissident groups outside of Burma urged the military junta yesterday to allow aid groups to operate freely in the wake of the cyclone — something it has been reluctant to do in the past.

“International expertise in dealing with natural disasters is urgently required. The military regime is ill-prepared to deal with the aftermath of the cyclone,” the secretary general of the Thailand-based forum, Naing Aung, said.

The storm’s 120 mph winds blew the roofs off hospitals and cut electricity to the country’s largest city.

The top American diplomat in Yangon, Shari Villarosa, said the storm’s whipping winds and torrential downpour had caused “major devastation throughout the city.”

“The Burmese are saying they have never seen anything like this, ever,” Ms. Villarosa told the Associated Press. “Trees are down. Electricity lines are down. Our Burmese staff have lost their roofs.”

At least 351 people were killed, including 162 who lived on Haing Gyi island off the country’s southwest coast, military-run Myaddy television station reported. Many of the others died in the low-lying Irrawaddy delta.

“The Irrawaddy delta was hit extremely hard not only because of the wind and rain but because of the storm surge,” the United Nation’s acting humanitarian coordinator in Yangon, Chris Kaye, said. “The villages there have reportedly been completely flattened.”

State television reported that in the Irrawaddy’s Labutta township, 75% of the buildings had collapsed.

The United Nations planned to send teams today to assess the damage, Mr. Kaye said. Initial assessment efforts have been hampered by roads clogged with debris and downed phone lines, he said.