It could cause radiation to leak out across the region, including into China, Wang Naiyan, of the China Nuclear Society, told the South China Morning Post. The increased size of the bombs made the risk of blowing the top off the mountain more likely, he added.
The paper said geophysicists at the University of Science and Technology of China in Anhui province had pinpointed all five blasts to the Punggye-ri area.
The US ambassador for the United Nations, Nikki Haley, said North Korea's leader Kim Jong-un was "begging for war" after the latest nuclear test on Sunday, the regime's biggest yet.
Pyongyang claimed to have detonated a thermonuclear warhead which could be fitted onto an intercontinental ballistic missile (ICMB). Then on Tuesday morning the North was reportedly seen moving a rocket resembling an ICBM towards its west coast.
The test resulted in South Korea boosting its own military capabilities. Washington and Seoul agreed to lift restrictions on South Korean missiles, according to the South Korean presidential office, allowing Seoul to improve its pre-emptive strike capabilities against the North.
Donald Trump has agreed with South Korea's leader, Moon Jae-in, to "maximise pressure" on Pyongyang.
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