Barack Obama warns North Korea that US 'will not hesitate to use our military might' to defend allies

US President Barack Obama's visit to Seoul came as North Korea threatened to conduct its fourth nuclear test, leading Mr Obama to raise the possibility of further sanctions.
"The commitment that the United States of America has made to the security of the Republic of Korea only grows stronger in the face of aggression," Mr Obama said in a speech to some of the 28,000 US service members stationed in South Korea to keep watch on its northern neighbour.
"Our alliance does not waiver with each bout of their attention seeking. It just gains the support of the rest of the world," Mr Obama said.
The website 38 North, which closely monitors North Korea, said commercial satellite imagery from Wednesday showed increased movement of vehicles and materials near what are believed to be the entrances to two completed tunnels at Punggye-ri nuclear test site.
The movements could be preparations for an underground atomic explosion, although predicting underground tests is notoriously difficult.