A construction worker said how he cheated death when a Bangkok skyscraper collapsed "in the blink of an eye" after a massive earthquake hit Myanmar and Thailand.

Tearful family members gathered at the remains of the 30-storey building, which crumbled to rubble in just seconds on Friday, clinging to shreds of hope that their loved ones who were working when it fell might be found alive.

The tower was being built to house government offices when the quake struck, and construction worker Khin Aung told AFP how the building collapsed just after his brother had entered to start his shift.

"When my shift ended around 1:00 pm I went outside to get water and I saw my younger brother before I went out," he told AFP on Saturday.

Tremors from the 7.7-magnitude quake centred in neighbouring Myanmar -- where the ruling junta said at least 694 people had died -- hit Bangkok around 1:20 pm (0620 GMT), shaking the building.

"When I went outside, I saw dust everywhere, and I just ran to escape from the collapsing building," Khin Aung said.

"I video-called my brother and friends but only one picked up the phone. But I can't see his face and I heard he was running.

"At that point, the whole building was shaking, but while I was on a call with him, I lost the call, and the building collapsed."

Authorities say up to 100 workers may be trapped in the mass of rubble and twisted metal that is all that remains of the tower. At least five are confirmed dead, but the toll is almost certain to rise.

"I can't describe how I feel -- it happened in the blink of an eye," said Khin Aung.

"All my friends and my brother were in the building when it collapsed. I don't have any words to say."

- Desperate relatives -

Bangkok's skyline is ever-changing, with buildings constantly torn down and shiny new skyscrapers thrown up.

The ceaseless reinvention is powered by an army of labourers, a huge proportion of whom are drawn from Myanmar by the prospect of regular work, a peaceful country and better wages than at home.

Many relatives of workers from Myanmar gathered at the site on Saturday, hoping for news of the missing.

Khin Aung and his brother -- married with two children -- have been working in Bangkok for six months.

"I heard they sent 20 workers to the hospital, but I don't know who they are and my friends and brother are among them," he said.

"I hope my brother and friends are in the hospital. If they are at the hospital, I have hope. If they are under this building, there is no hope for them to survive."

Thai woman Chanpen Kaewnoi, 39, waited anxiously for news of her mother and sister, who were in the building when it went down.

"My colleague called and said she couldn't find my mum or my sister. I thought mum might have slipped, and maybe my sister stayed to help her," she told AFP.

"I want to see them, I hope I can find them. I hope they will not be lost. I still have hope, 50 percent."

As distraught families waited for news, rescue workers pressed on with the delicate task of searching the ruins without triggering further collapses.

The death toll from a massive earthquake in Myanmar has passed 1,000, the ruling junta said on Saturday, with more than 2,000 injured.

A statement from the junta's information team said 1,002 people are known to have died in Friday's shallow 7.7-magnitude quake, with 2,376 injured.

AFP