The US and South Korea have threatened North Korea with "appropriate" reactions after Pyongyang's apparent nuclear test. US President Barack Obama has also agreed to work together with Japan.
South Korea's Defense Ministry in Seoul said on Thursday that a phone call had taken place between US Defense Secretary Ash Carter and his South Korean counterpart Han Min Koo.
Both sides wanted to coordinate "appropriate" reactions to North Korea's provocation, the ministry said. Exactly what those reactions would involve was not immediately clear.
According to the White House, President Barack Obama also talked with Japanese Prime Minister Shinzo Abe on Thursday about the international response to North Korea's apparent nuclear test.
Discussions between the US and Asian leaders on Thursday came just a day after North Korea claimed it had "successfully" tested a hydrogen bomb for the first time. Initial reports described the test's tremor as a "man-made" earthquake.
The self-declared H-bomb test, two days before North Korean leader Kim Jong Un's birthday, was the fourth time the isolated state had exploded a nuclear device in the last 10 years.
In response to the claims, the UN Security Council agreed on Wednesday to also implement new measures to punish North Korea for its provocation.
Although experts remain skeptical as to whether North Korea really has H-bomb capability, the announcement from the North has reignited fears in the international community over Pyongyang's efforts to build a warhead small enough to be mounted on a missile and capable of reaching the shores of the mainland US.
In the past, North Korean officials have threatened to target the US mainland and destroy South Korea in a "sea of flames."
China key in next step
Analysts say China's response following the test will be key in determining the international community's next step.
Beijing has restrained US-led allies from stronger action against Pyongyang in the past, but has displayed increasing frustration with Pyongyang's refusal to suspend testing.
US Democratic presidential hopeful Hillary Clinton called on Beijing to be more assertive in deterring the North, also calling for sanctions and strengthened American missile defense.
ksb/rc (Reuters, AFP, AP)