Thailand has reported 15 new coronavirus deaths, setting a daily record for the third time in four days in its worst outbreak yet, as an expert warned of big problems ahead from insufficient testing.

“If we are not screening people who can quietly spread the virus, there will be subsequent outbreaks and mutations, rendering immunisation ineffective,” Thiravat Hemachudha, head of the Thai Red Cross Emerging Infectious Diseases Centre wrote on Facebook.

His comments come amid public criticism of the government for its slow response to the latest outbreak and delayed vaccine rollout, with some calling for the health minister's resignation.

Thailand has reported 30,824 infections and 69 deaths this month alone, against a total of 59,687 cases and 163 fatalities since the pandemic began last year, according to Reuters.