North Korea may be preparing for a fourth nuclear test, with stepped up activity detected at its main test site, a senior South Korean government official said according to a report Monday.
"We are closely monitoring the ongoing situation, which is very similar to the situation seen ahead of the third nuclear test," the unnamed official told the daily JoongAng Ilbo.
North Korea carried out its third nuclear test in February, drawing toughened UN sanctions and kickstarting a cycle of escalating tensions that has seen Pyongyang threaten nuclear strikes against the US and South Korea.
South Korean intelligence authorities have detected unusually busy personnel and vehicle movements at the Punggye-ri nuclear test site in the country's northeast since last week, the government official reportedly said.
"We are trying to figure out whether it is a genuine preparation for a nuclear test or just a ploy to heap more pressure on us and the US," the official added.
The heightened activity has been detected near a tunnel at the south side of the atomic test site, the official added.
The newspaper cited a senior military official as saying that the North was capable of staging another nuclear test "within several days once a political decision is made."
The North's third nuclear test -- its most powerful to date -- followed previous tests in 2006 and 2009.
Pyongyang also said last week that it would resume operations at its mothballed Yongbyon nuclear complex, its only source of weapons-grade plutonium.