North Korea's "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.

Kim Jong Un's regime is battling a spiralling Covid-19 outbreak with nearly two million reported cases of "fever", but both Washington and Seoul say this has not derailed Pyongyang's test plans.

After a record-breaking blitz of missile launches this year, including intercontinental ballistic missiles, Kim could seek to distract North Koreans from the spiralling health crisis with a nuclear weapons test, analysts say.

US intelligence thinks there is a "genuine possibility" that Kim could choose to stage a "provocation" after Biden arrives in Seoul late Friday for his first trip as president to Asia, his administration said.

This could mean "further missile tests, long range missile tests or a nuclear test, or frankly both" around the time of Biden's trip, National Security Advisor Jake Sullivan said.

Satellite imagery also indicates North Korea is preparing to conduct what would be its seventh nuclear test.

Both Seoul and Washington have been warning for weeks that it could come any day.

- No stopping them? -

"North Korea will want to attract global attention by conducting a nuclear test during President Biden's visit to South Korea and Japan," Cheong Seong-chang of the Center for North Korea Studies at the Sejong Institute told AFP.

Leader Kim is likely debating what to do, with the United States pressuring North Korea's sole major ally China to help rein in Pyongyang's nuclear and ICBM tests.

But with Kim well aware of the gridlock at the UN Security Council after Russia's invasion of Ukraine, "it seems highly likely that he will carry out nuclear or ICBM tests during President Biden's visit to South Korea or Japan," Cheong added.

Biden lands in South Korea on Friday for his first summit with the country's new president Yoon Suk-yeol, who took office last week.

Talks between Washington and Pyongyang have been stalled for years, after an extraordinary show of diplomacy between then US president Donald Trump and Kim -- which was brokered by Yoon's predecessor Moon Jae-in -- ultimately ended in failure.

Trump held three headline-grabbing meetings with Kim and has claimed that the two were "in love", but analysts say little to no progress was made in dismantling the North's nuclear programmes.

Kim recently said he was strengthening his nuclear arsenal "at the fastest possible speed".

Part of the blame for the state of play must be laid at Biden's door due to his "strategic neglect" of North Korea since he took office, said Park Won-gon, a professor at Ewha University.

"In terms of denuclearisation and US-North Korea ties, we have returned to a situation where it's difficult to find any progress," Park said.

"There is no way to really stop North Korea now," he said.