Officials in the northwestern US state of Washington on Wednesday expressed concern over reports of people organising "Covid-19 parties" to intentionally spread the virus.

"Gathering in groups in the midst of this pandemic can be incredibly dangerous and puts people at increased risk for hospitalisation and even death," warned John Wiesman, the state's secretary of health.

"Furthermore, it is unknown if people who recover from Covid-19 have long-term protection," he said.

"There is still a lot we don't know about this virus, including any long-term health issues which may occur after infection."

Wiesman's comments came after officials in Walla Walla County, located 260 miles (420 kilometres) southeast of Seattle, reported that some of the nearly 100 cases in the region appear to have been intentionally spread or contracted at so-called "Covid-19 parties."

The aim of these gatherings is for non-infected people to mingle with an infected person in an effort to catch the virus.

"This kind of unnecessary behavior may create a preventable uptick in cases which further slows our state's ability to gradually re-open," Wiesman said.

As of Wednesday, there were 94 cases of coronavirus reported in Walla Walla county and one death.

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Meghan DeBolt, Walla Walla's community health director, said contact tracing had shown that some of those infected had attended parties with the aim of contracting the virus.

"We don't know when it is happening. It's after the fact that we hear from cases," she told the Walla Walla Union-Bulletin. "We ask about contacts, and there are 25 people because: ‘We were at a Covid party.'"

She said such behavior was irresponsible and urged residents in a Facebook message to follow proper physical and social distancing measures to prevent community transmission.

"We need to use this time to use good common sense and to be smart as we move through this pandemic so that we can begin to reopen our community," she said. "Covid-19 parties: not part of the solution," she added.

There has so far been only one other report in the United States of a coronavirus party.

In March, Kentucky's Governor Andy Beshear announced that a person had contracted the virus after attending a Covid-19 party.

The United States is the country hardest hit by the pandemic with more than 1.2 million cases so far and 73,095 deaths.