A new study by the US National Institutes of Health (NIH) has suggested that COVID-19 cases were detected in the country earlier than previously reported.

NIH researchers analysed more than 24,000 stored blood samples from all 50 US states between January 2 and March 18, 2020, Xinhua news agency quoted the study as saying.

They detected antibodies against SARS-CoV-2, the virus that caused COVID-19, using two different serology tests in nine participants' samples.

Seven of those samples were seropositive prior to the first confirmed case in the states of Illinois, Massachusetts, Wisconsin, Pennsylvania, and Mississippi, according to the study published by the Clinical Infectious Diseases journal.

The study is the latest to suggest the coronavirus first appeared in the US earlier than previously known.

The US continues to be the worst-hit country with the world's highest number of cases and deaths at 33,498,145 and 600,648, respectively, according to the Johns Hopkins University.