Joe Biden and his South Korean counterpart, Yoon Suk-yeol, have said they are considering expanding joint military exercises in response to the “threat” posed by North Korea, a move that is expected to enrage the regime as speculation builds that it could conduct a nuclear test. Speaking in Seoul on the second day of his visit to South Korea, Biden said he was willing to meet North Korea’s leader, Kim Jong-un, but only if he was “sincere and serious” about dismantling his nuclear and ballistic missile programmes. Yoon, a conservative who took office just 11 days ago, said he and Biden also discussed the timing of a possible deployment in the South of US “strategic assets” – a term that typically refers to bombers, submarines and aircraft carriers – to deter North Korea. The two measures, if implemented, would mark an escalation in US and South Korean efforts to put on a show of force against Pyongyang, and an admission that nuclear talks started by Donald Trump are unlikely to be revived. Some intelligence officials believe the North could be preparing to conduct a nuclear test or ballistic missile launch – or possibly both – to coincide with Biden’s five-day visit to South Korea and Japan. But the US president also struck a conciliatory note, repeating his offer to provide the North with vaccines as it battles an outbreak of Covid-19 that has made 2.4 million people sick and killed at least 66. “Yes, we’ve offered vaccines, not only to North Korea, but China as well,” Biden said at a joint news conference with Yoon. “We’re prepared to do that immediately. We’ve gotten no response.” The leaders said in a statement that their shared goal was the complete denuclearisation of the Korean peninsula – a loose commitment Kim made at his first summit with Trump in 2018. But talks have been stalled since early 2019, when Trump and Kim failed to agree on how moves to dismantle the regime’s nuclear arsenal should be met with sanctions relief. Biden and Yoon said they were committed to a “rules-based international order” in the wake of the Russian invasion of Ukraine – wording that could provoke North Korea, which describes its nuclear arsenal as a deterrent against US “aggression”. The country has long condemned joint US-South Korea war games as a rehearsal for invasion, while the allies insist they are purely defensive in nature. Yoon said he and Biden were in “lockstep” on South Korea’s security in the face of an increasingly unpredictable North. “There can be no compromise on security,” he said. “The door to dialogue remains open if North Korea takes genuine steps towards denuclearisation,” he said, adding that the regime would be rewarded with an “audacious” package of economic investment. There is, however, unease in Seoul over how much energy Biden is prepared to commit to the Korean peninsula when his focus is on his domestic agenda, midterm elections and the war in Ukraine. Biden only touched on North Korea in his remarks on Saturday, and some worry that the US could return to the days of “strategic patience”, when Barack Obama waited for the regime in Pyongyang to demonstrate it was serious about discussing its nuclear arsenal. That laissez-faire approach has been blamed for giving Pyongyang the latitude to build a nuclear deterrent, despite years of UN-led sanctions. Speculation that international cooperation to address the Covid-19 outbreak in North Korea could help ease tensions and encourage dialogue has so far proved misplaced. The regime has ignored offers of medical assistance from the South and the US, and continues to shun the UN-backed Covax vaccination programme. However, reports suggest it is receiving medical supplies from China, its biggest trading partner. “We are very concerned about the Covid situation,” a senior US administration official said on Saturday. “We are very sensitive to the fact that they appear to be facing a quite serious situation, and I think you’ve seen we stand ready to work with others in the international community as needed to provide assistance.” North Korea on Saturday reported almost 220,000 more cases of “fever” and one death, bringing the death toll to 66 since the regime admitted for the first time it was battling a Covid outbreak this month. But experts say the caseload and death toll is almost certainly higher. Biden began Saturday by laying a wreath at a cemetery in Seoul for South Korean soldiers killed during the 1950-53 Korean war, including those who fought alongside US forces. But he will not visit the demilitarised zone, the heavily armed border that separates North and South Korea. Previous US presidents have visited the site, and Biden went as vice-president in 2013.