North Korea threat has US on alert

Tensions have heightened following the North's underground nuclear test and firing of short-range missiles this week.

In response, Seoul joined more than 90 nations that have agreed to stop and inspect vessels suspected of transporting banned weapons.

North Korea claims the move is a prelude to a naval blockade and raises the prospect of naval skirmish in its western waters.

The South Korea-US combined forces command has raised its surveillance from level three to level two.