Samples from US wastewater sites have shown an increase in the coronavirus this month in a third of locations, the Centers for Disease Control and Prevention (CDC) confirmed, suggesting that cases in key cities including New York are rising.

The rise in the virus that causes COVID-19 was detected in wastewater samples collected from March 1-10, the CDC's COVID data tracker showed. At least one-third or 36 percent of samples from wastewater sites showed an increase in the presence of the virus, according to Blomberg. It was more than twice the levels reported from Feb 1-10.

The CDC confirmed on Tuesday that there had been an increase in the virus in wastewater samples. But COVID-19 cases remain relatively low across the US when compared with the same time last year. At least 81 percent of Americans have received at least one dose of a COVID-19 vaccine. It has led several cities including New York and Los Angeles to drop mask mandates.

Current data from the CDC shows that between Feb 26 and March 12, out of 398 wastewater sites collecting data, 44 saw an increase of 1,000 percent or more of the virus, and 62 sites saw an increase of 100 percent or more.

Wastewater samples provide important, advanced information about the spread of coronavirus because those who are infected shed the virus in their feces. The agency added wastewater data to its COVID-19 tracker on Feb 4.

Dr Rochelle Walensky, CDC director, said during a White House press briefing on Feb 9: "This is a powerful tool that, when paired with traditional public health surveillance, can help us identify where the disease is spreading and how best to distribute resources."

Dr John Dennehy, a professor of biology at Queens College of the City University of New York researches virus evolution, ecology and emerging viruses in new host populations.

Dennehy told China Daily: "In cities with sewers, everyone contributes to the wastewater, thus wastewater provides an unbiased snapshot of SARS-CoV-2 prevalence."

However, the increase of the virus detected in wastewater samples comes amid fears that a new coronavirus variant BA.2 could be more virulent than the highly transmissible Omicron variant.

BA.2 made up a quarter (23.1 percent) of the variant spreading in the US as of March 12 in the US, the CDC said.

Dr Maria Van Kerkhove, the World Health Organization's technical lead for COVID-19 said BA.2 could be the most transmissible variant yet. It is said to be 80 percent more virulent than Omicron.

BA.2 now makes up 39 percent of total cases in New Jersey, New York, Puerto Rico and the Virgin Islands. It is 38.6 percent of cases in states such as Connecticut, Maine, Massachusetts, New Hampshire, Vermont and Rhode Island, Reuters reported.

In European nations, there was a 2 percent rise in cases but no uptick in deaths. In African nations, there was a 12 percent rise in new cases and a 14 percent rise in deaths.

Another set of Omicron subvariants of concern are the BA.1.1. and the B.1.1.529. They first began spreading in December and make up over 66.1 percent and 10.8 percent of variants in the US, Reuters reported.

"These increases are occurring despite reductions in testing in some countries, which means the cases we're seeing are just the tip of the iceberg," WHO Director-General Tedros Adhanom Ghebreyesus told reporters on March 16.

Globally, new infections have risen by 8 percent compared with the previous week.

Worldwide, there were 11 million new cases and just over 43,000 new deaths reported from March 7-March 13, the first rise since the end of January.