After reports of some COVID-19 patients testing positive for the disease after initially testing negative emerged, the World Health Organization (WHO) has decided to investigate the issue.

In South Korea, about 91 patients, who were thought to have recovered from COVID-19 again tested positive. Earlier similar reports came from other parts of the world including China, giving rise to a new theory that the virus could be reactivated.

This prompted the UN health agency on Saturday to confirm that it would look into the concerns.

"We are aware of these reports of individuals who have tested negative for COVID-19 using PCR (polymerase chain reaction) testing and then after some days testing positive again," The Hill quoted the WHO as saying in a statement.

"We are closely liaising with our clinical experts and working hard to get more information on those individual cases. It is important to make sure that when samples are collected for testing on suspected patients, procedures are followed properly," it added.

As per the clinical management guidelines from the WHO, a COVID-19 patient who has tested negative twice in tests at least 24 hours apart can be discharged from hospital.

"We are aware that some patients are PCR positive after they clinically recover, but we need systematic collection of samples from recovered patients to better understand how long they shed live virus," the WHO said.

Worldwide, more than 1.7 million people have been infected by the virus out of which over 109,000 have died.