Speaking on a visit to Seoul, the US President Joe Biden expressed concerns about the North's growing nuclear capabilities and said he would meet its leader Kim Jong Un if "he was serious".

Mr Biden was in the country for his first diplomatic talks with his new South Korean counterpart Yoon Suk Yeol, who was inaugurated 11 days ago.

Dominating the discussions were continuing tensions with North Korea and fears it could launch fresh missile tests.

In a joint statement, Mr Biden reaffirmed the US commitment to defend South Korea with nuclear weapons if necessary.

The two sides agreed to consider expanding their combined military drills, which had been scaled back in recent years in an effort to ease fraught relations with the North.

The prospect of bigger exercises between the allies risks an angry response from North Korea.

Pyongyang has defended its nuclear weapons and missile development on what it sees as US threats and has long described the drills as invasion rehearsals, although the US and South Korea maintain they are defensive.

The US also promised to deploy, if needed, "strategic assets" which typically include long-range bomber aircraft, submarines, or aircraft carriers.

Both leaders insisted they were open to diplomacy with Pyongyang.

Mr Biden said: "With regard to whether I would meet with the leader of North Korea, it would depend on whether he was sincere and whether he was serious."

He also revealed Washington had offered COVID-19 vaccines to China and North Korea, which is tackling its first acknowledged outbreak.

"We've got no response," Mr Biden said.

Seoul had also offered assistance to the North that has so far been ignored.

While North Korea reported more than 200,000 new patients suffering from "fever" for a fifth consecutive day on Saturday, the country has little in the way of vaccines or modern treatment for the pandemic.

The snub to offers of coronavirus assistance has reduced hopes that such cooperation could help ease nuclear tensions or even lead to talks.

So far this year North Korea has tested missiles 16 separate times, including in March, when its first launch of an intercontinental ballistic rocket since 2017 demonstrated a potential range including the entire US mainland.

The Biden administration has called on China to restrain Pyongyang from engaging in any further tests.