Barack Obama has warned North Korea the United States "will not hesitate to use [its] military might" to defend its allies.

The US president made a speech in Seoul, South Korea, showcasing America's power in the region amid China's growing influence and threats from Pyongyang.

The visit came as North Korea threatened to conduct its fourth nuclear test, leading Obama to raise the possibility of further sanctions. Obama told some of the 28,000 American service members stationed in South Korea to keep watch on its northern neighbour.

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