“Preparations for a nuclear test have been completed and they are only looking for the right time”, Ha Tae-keung said after attending a Seoul's National Intelligence Service briefing.
The US had earlier sounded the alarm on North Korea’s nuclear ambitions, saying there is a “genuine possibility” that it could conduct the tests on Mr Biden’s first trip as president to Asia, where he will be attending 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 adviser Jake Sullivan said.
The White House said Mr Biden is not expected to visit the Demilitarised Zone between North and South Korea during his trip.
North Korea is in the middle of a raging Covid-19 outbreak, with hundreds of thousands of cases being reported daily.
“Our intelligence does reflect a genuine possibility that there will be either a further missile test, including long-range missile test, or a nuclear test, or frankly both, in the days leading into, on, or after the president's trip to the region,” Mr Sullivan told a White House briefing on Wednesday.
“We are preparing for all contingencies,” he said.
Mr Biden’s five-day trip will include a summit with South Korea’s new President Yoon Suk-yeol, who assumed office on May 10. Unlike his predecessor, Mr Yoon has taken a more hardline stance against his northern neighbour's provocations.