A huge earthquake hit Myanmar and Thailand on Friday, killing more than 150 people and injuring hundreds, with dozens trapped in collapsed buildings and the death toll expected to rise.

The shallow 7.7-magnitude tremor hit northwest of the city of Sagaing in central Myanmar in the early afternoon, and was followed minutes later by a 6.4-magnitude aftershock.

The quake flattened buildings, downed bridges, and cracked roads across swathes of Myanmar, and even demolished a 30-storey skyscraper under construction hundreds of kilometres (miles) away in Bangkok.

While the full extent of the catastrophe is yet to emerge, the leader of isolated Myanmar, in the grip of a civil war, issued a rare plea for international aid.

Junta chief Min Aung Hlaing said 144 people had been killed, with 732 confirmed injured, but warned the toll was "likely to rise". Eight deaths have been confirmed so far in Thailand, with more expected.

"In some places, some buildings collapsed," he said in a televised speech, after visiting a hospital in the capital Naypyidaw.

"I would like to invite any country, any organisation, or anyone in Myanmar to come and help. Thank you."

He urged massive relief efforts in the wake of the disaster and said he had "opened all ways for foreign aid".

US President Donald Trump said Washington would be offering assistance.

'Mass casualty area'

Four years of civil war sparked by the military seizing power have ravaged Myanmar's infrastructure and healthcare system, leaving it ill-equipped to respond to such a disaster.

Power is down in parts of Myanmar's biggest city Yangon, the local utility said.

The country declared a state of emergency across the six worst-affected regions after the quake, which the World Health Organization described as a "very, very big threat to life and health".

Hundreds of casualties arrived at a major hospital in Naypyidaw, where the emergency department entrance had collapsed on a car.

Medics treated the wounded outside the hospital, the same one visited by the junta chief. One official described it as a "mass casualty area".

"I haven't seen (something) like this before. We are trying to handle the situation. I'm so exhausted now," a doctor told AFP.

Mandalay, Myanmar's second largest city, appeared to have been badly hit. AFP photos from the city showed multiple buildings in ruins.

A resident reached by phone told AFP that a hospital and a hotel had been destroyed, among other buildings, and said the city was badly lacking in rescue personnel.

The main road bridge linking Mandalay and Sagaing was down, the city's university and historic palace wall have collapsed, and telecoms have been affected, the International Federation of Red Cross and Red Crescent Societies said.

Skyscraper collapse

Across the border in Thailand, a 30-storey skyscraper under construction collapsed to a tangled heap of rubble and dust in a matter of seconds.

Interior Minister Anutin Charnvirakul said eight dead bodies have been recovered and, with between 90 and 110 people unaccounted for, the toll is expected to rise.

"We see several dead bodies under the rubble. We will take time to bring the bodies out to avoid any further collapses," he told reporters.

"I heard people calling for help, saying 'help me'," Worapat Sukthai, deputy police chief of Bang Sue district, told AFP.

As night fell, around 100 rescue workers assembled at the scene to search for survivors, illuminated by specially erected floodlights.

Visiting the site, Prime Minister Paetongtarn Shinawatra said "every building" in Bangkok would need to be inspected for safety, though it was not immediately clear how that would be carried out.

An emergency zone was declared in Bangkok, where some metro and light rail services were suspended.

The streets of the capital were full of commuters attempting to walk home, or simply taking refuge in the entrances of malls and office buildings.

City authorities said parks would stay open overnight for those unable to sleep at home.

Strong quakes are extremely rare in Thailand, and across Bangkok and the northern tourist destination of Chiang Mai, where the power briefly went out, stunned residents hurried outside, unsure of how to respond.

"This is the strongest tremor I've experienced in my life," said Sai, 76, who rushed out of a minimart in Chiang Mai when it started to shake.

The quake was felt across the region, with China, Cambodia, Bangladesh and India all reporting tremors.

India, France and the European Union all offered to provide assistance, while the WHO said it was mobilizing its logistics hub in Dubai to prepare trauma injury supplies.

Pope Francis said he was "deeply saddened by the loss of life and widespread devastation" in a telegram published by the Vatican.

Earthquakes are relatively common in Myanmar, where six strong quakes of 7.0 magnitude or more struck between 1930 and 1956 near the Sagaing Fault, which runs north to south through the centre of the country, according to the United States Geological Survey (USGS).

A powerful 6.8-magnitude earthquake in the ancient capital Bagan in central Myanmar killed three people in 2016, also toppling spires and crumbling temple walls at the tourist destination.