North Korea test-fired two short-range ballistic missiles on Saturday, neighbouring countries said.
It is the fourth round of weapons launches that prompted quick, strong condemnation from its rivals, this week.
In a strong rebuke, South Korean President Yoon Suk Yeol said North Korea's "obsession" with nuclear weapons is deepening the suffering of its own people and warned of an "overwhelming response" from South Korean and US officials should such weapons be utilised.
Mr Yoon said: "North Korea hasn't abandoned its obsession with nukes and missiles despite the persistent international objection in the past 30 years.
"The development of nuclear weapons will plunge the lives of North Korean people in further pains.
"If North Korea attempts to use nukes, it'll face a resolute, overwhelming response by the South Korea-US alliance and our military."
The comments could anger North Korean leader Kim Jong Un, who in July alleged that Mr Yoon's government was led by "confrontation maniacs" and "gangsters".
This week's North Korean testing is seen as a response to recent US-South Korean naval drills and their other training that involved Japan.
North Korea sees such military exercises by the allies as an invasion rehearsal and argues they reveal US and South Korean "double standards" because they brand the North's weapons tests as provocation.
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On Saturday, South Korea, Japanese and US militaries said they detected the two North Korean missile launches. South Korean officials said the lift-offs occurred from North Korea's capital region.
According to South Korean and Japanese estimates, the missiles flew between 220 and 250 miles at a maximum altitude of 20-30 miles before they landed in the waters between the Korean Peninsula and Japan.
Toshiro Ino, Japan's vice defence minister, said the missiles showed "irregular" trajectory.
Some observers say the weapons' reported low and "irregular" trajectory suggest they were likely nuclear-capable missiles modelled after Russia's Iskander missile.
They say North Korea has developed the Iskander-like weapon to defeat South Korean and US missile defences, and strike key targets in South Korea, including US military bases.