A total of 21 US states on Tuesday reported weekly increases in new cases of COVID-19, with the southwestern corner of the country apparently worst-hit. The surge comes amid the US easing virus restrictions in a bid to restart its economy.

Cases in Arizona — one of the first states to reopen in mid-May — have increased 115% since the lockdown eased. A former state health chief warned that a new stay-at-home order or field hospitals may be needed.

Utah and New Mexico all posted rises of 40% or higher for the week ended Sunday, compared with the prior seven days, according to a Reuters analysis.

US non-profit health system Banner Health tweeted that the trend was "concerning."

California sees rise in several counties

California has also seen an uptick in cases and has placed nine counties on a watch list, meaning harsher coronavirus measures could be reinstated. The counties the list include Los Angeles, Santa Clara and Fresno  ­­— where 18 million of California's 39 million residents live, reported Reuters.

Read more: Opinion: New York — A stricken city of contrasts

 "Many of the cases that are showing up in hospitals are linked to gatherings that are taking place in homes — birthday parties and funerals," said Olivia Kasirye, public health director of Sacramento County, also on the watch list.

So far, across the US there have been over 1.9 million cases and over 112,000 deaths from the virus  — by far, the largest death toll worldwide. However, the Trump administration has been eager to loosen coronavirus restrictions in a bid to help its economy, which has taken a battering.

Earlier this week, non-partisan research organization the National Bureau of Economic Research said the US officially entered recession in February this year. The coronavirus measures also caused record unemployment levels, reaching an all-time high of 14.7% in April, although the latest figures showed this decreased to 13.3% in May.

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