Four more bodies have been found in the rubble that was the State Audit Office tower in Bangkok, as searchers approach an area where many labourers were working when the March 28 earthquake struck.

Bangkok deputy governor Tavida Kamolvej said on Tuesday the discovery of the four bodies on Monday night raised the confirmed death toll to 21, with 73 people still unaccounted for. 

The unfinished 30-floor building in Chatuchak district collapsed to the ground during the strong earthquake, which was centred near Mandalay in Myanmar. 

The deputy governor said backhoes were working in two areas of the huge pile of debris where 20 to 30 people were working when the building collapsed.

Most of the workers onsite were from Myanmar.

No other high-rise in the city suffered serious damage. There have been reports suggesting the use of sub-standard materials in the building's construction. An investigation is underway.