South Korean President Park Geun-hye on Monday said North Korea appeared to be readying for a fifth nuclear test in defiance of tightened United Nations sanctions imposed after its last test in January.
"Signs that it is preparing a fifth nuclear test have recently been detected," Park told a cabinet meeting.
Her remarks followed South Korean media reports in which unnamed government and intelligence officials spoke of a spike in activity at the North's Punggye-ri nuclear test site.
A fresh test would see Pyongyang doubling down in the face of tough sanctions adopted by the UN Security Council, and would throw down a gauntlet to the international community as it struggles to find new ways to curb the North's nuclear ambitions.
The United States will respond strongly in the case of a further North Korean nuclear test, US Deputy Secretary of State Antony Blinken said on Monday.
Blinken, who met senior Japanese government officials in Tokyo, told reporters North Korea would be digging deeper into a hole if it pursued further provocations.
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North Korea is gearing up for a rare and much-hyped ruling party congress early next month, at which leader Kim Jong-un is expected to take credit for pushing the country's nuclear weapons programme to new heights.
Numerous analysts have suggested that the regime might carry out a fifth nuclear test as a display of defiance and strength just before the congress opens.
"It is uncertain what kind of unexpected provocations it would stage," Park said at the cabinet meeting.
The South Korean Defence Ministry said it was alert to the likelihood of a fifth test.
"Given current activities, we believe that there is a possibility that the North may stage an underground nuclear test, and are monitoring the situation accordingly," ministry spokesman Moon Sang-gyun told reporters, according to the AFP news agency.
South Korea's vice foreign minister will discuss the North Korean threat during trilateral talks this week with his US and Japanese counterparts in Seoul.
Tension has been high on the divided Korean peninsula since the January test and a rocket launch a month later that was widely seen as a disguised ballistic missile test.
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The UN Security Council responded by imposing its strongest sanctions to date over the North's nuclear weapons programme.
Pyongyang has responded defiantly, staging a series of short and mid-range missile tests and claiming a series of significant technical breakthroughs in its nuclear strike capability.
It claimed it had miniaturised a nuclear warhead to fit on a missile and successfully tested an engine designed for an inter-continental ballistic missile that could reach the mainland US.
While some analysts say the claims are exaggerated, most acknowledge that the North's nuclear and ballistic missile programmes have made significant strides.
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