North Korea claims first tested hydrogen nuclear device.

North Korea said it had successfully conducted a test of a miniaturized hydrogen nuclear device on Wednesday morning.

The announcement on North Korean state TV followed detection of a 5.1 magnitude earthquake near its known nuclear test site earlier on Wednesday.

The nuclear test is the fourth by the isolated country, which is under U.S. and UN sanctions for its nuclear and missile programs.

International experts say there is no confirmation of this yet.

But the announcement was swiftly criticized by world leaders, with South Korea calling it "a grave provocation to our national security".