Naypyidaw, Myanmar - 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-story skyscraper under construction hundreds of 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.

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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 organization, 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".

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.

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.

An official at the hospital, the same one visited by the junta chief, described it as a "mass casualty area" with medics treating the wounded outside.

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