South Korean President Yoon Suk-yeol has reaffirmed his resolve to strengthen his country's security alliance with the United States to counter the threat posed by North Korea's nuclear and missile programs.

Yoon spoke during a ceremony at Seoul National Cemetery on Tuesday marking Memorial Day to commemorate those who lost their lives in the Korean War.

Yoon noted that prior to the ceremony he had attended the burial of a fallen soldier whose remains were recently discovered. He then expressed his respect to the war dead.

Yoon noted that the US has declared it would strengthen its extended deterrence -- Washington's commitment to defend its allies with nuclear weapons and other capabilities.

He said this means South Korea's alliance with the US has now been upgraded to a "nuclear-based alliance."

Yoon stressed that he will defend people's lives and safety by building an ironclad security posture based on a stronger-than-ever alliance with the US.

North Korea has vowed to make a new attempt to launch a military reconnaissance satellite after its initial effort failed last week.

The US and South Korean militaries are on alert. They have been staging massive live-fire drills since late last month.