The 30-storey skyscraper, set to house government offices, was the only major building to collapse in Bangkok when a 7.7 magnitude earthquake struck neighbouring Myanmar Friday, with tremors felt as far as the Thai capital.

Thai authorities say they are launching a probe into the construction firms responsible amid suspicions they used substandard steel rebar in the structure.

The development near Bangkok's popular Chatuchak market was a joint project involving China Railway No. 10 Engineering Group (Thailand) -- an offshoot of China Railway Group (CREC), one of the world's largest construction and engineering contractors.

AFP was unable to reach the Chinese company involved in the project for comment.

The Chinese embassy in Bangkok late Tuesday expressed "deepest condolences" for the collapse and urged firms to "strictly" comply with Thai laws.

"The Chinese government has consistently urged Chinese companies overseas to abide by local laws and contribute positively to the society," the embassy said in a Facebook post only in Thai.

Beijing has dispatched a team of rescue experts and disaster relief personnel to assist in Bangkok and vowed to "continue supporting Thailand as needed", it added.

It has also urged Chinese firms to cooperate "strictly" with Thai authorities as they investigate the collapse.

The local partner in the project, Italian-Thai Development (ITD), offered condolences on Monday to quake victims but said it was "confident" the incident would not impact its other projects.

The collapse was the deadliest single incident in Thailand from the earthquake, with the majority of the kingdom's 22 fatalities thought to be workers on the building site.