While visiting South Korea, US President Joe Biden said Washington and its allies were "prepared for anything," including Pyongyang restarting nuclear testing.

The leaders of the US and South Korea are "prepared for anything North Korea does," US President Joe Biden said on Sunday, amid rumors that North Korea could test another nuclear weapon after a five-year break.

"We've talked through how we'd respond to whatever they do so I am not concerned, if that's what you're suggesting," Biden told a reporter at a joint press conference with South Korea's newly-elected President Yoon Suk Yeol.

The discussion also included starting "joint drills to prepare for a nuclear attack," Yoon said.

Message for Kim Jong Un is 'Hello'

On the second day of Biden's visit to South Korea, the US president also visited Osan Air Base where US and South Korean troops are monitoring potential threats from Pyongyang.

When asked by a reporter if he had a message for the North Korean dictator Kim Jong Un, Biden said his message was "Hello. Period."

North Korea has amped up its missile tests in recent months, largely ignoring warnings from the West. The White House previously warned that the North Korean regime might test missiles or weapons during Biden's Asian tour as a "provocation."

Hyundai promises to fund electric car factory in Georgia

Biden also met with the head of the South Korea's car-making giant Hyundai as a part of his effort to deepen economic ties between US companies and US allies in Asia. Hyundai has pledged to invest $10 billion (some €9.5 billion) in the US. More than half of the funds will go into an electric car factory in the US state of Georgia.

Previously, Biden toured a Samsung semi-conductor plant he says would serve as a model for a new Samsung facility in Texas.

With the US facing pressure from Russia and China, Biden said Asia was a key battleground in the worldwide "competition between democracies and autocracies."

"We talked in some length about the need for us to make this larger than just the United States, Japan, and Korea, but the entire Pacific and the South Pacific and Indo-Pacific. I think this is an opportunity," Biden said.

Biden to meet Quad leaders in Tokyo

The US president then flew to Japan where he is due to meet the leaders of India and Australia on Tuesday. Washington, New Delhi, Canberra and Tokyo are members of the Quad — a security alliance widely seen as a way for Washington to check China's growing influence.

South Korea's Yoon has hinted at working with the Quad more closely, although there were currently no plans for Seoul to join the alliance.