Despite North Korea's recent Covid-19 outbreak, "preparations for a nuclear test have been completed and they are only looking for the right time", lawmaker Ha Tae-keung told reporters after being briefed by Seoul's National Intelligence Service.

The United States said earlier it believes there is a "genuine possibility" that North Korea could conduct a nuclear test while Biden is on his first trip as president to Asia.

Biden will arrive in Seoul late Friday for a series of summits.

"Our intelligence does reflect the genuine possibility" of nuclear-capable missile tests or a nuclear weapon test around the time of Biden's trip, National Security Advisor Jake Sullivan said.

Satellite imagery indicates North Korea is preparing to conduct a nuclear test, and the United States and South Korea have been warning for weeks that it could come any day.

North Korea announced its first Covid-19 cases last week, and is now reporting hundreds of thousands of cases of "fever" daily, with analysts saying a test could help distract the regime from the outbreak.