An anti-corruption watchdog had flagged to authorities irregularities it found in the construction of a Bangkok skyscraper before it collapsed in an earthquake last week, killing at least 12 people, the head of the group told Reuters.

Initial tests of materials gathered at the site, meanwhile, indicated the presence of substandard steel among the wreckage of the 30-storey structure, according to industry ministry officials, who added that further analysis was required.

The government has announced an investigation into the cause of the collapse of the skyscraper, the only Bangkok building to completely crumble when a 7.7 magnitude quake struck central Myanmar on Friday and rattled neighbouring countries.