SEOUL (Reuters) – North Korea, facing international censure for this week’s nuclear test, threatened on Wednesday to attack the South after it joined a US-led plan to check vessels suspected of carrying equipment for weapons of mass destruction.

Adding to tensions in the region, South Korean media reported that Pyongyang had restarted a plant that makes plutonium that can be used in nuclear bombs.

In Moscow, news agencies quoted an official as saying that Russia is taking precautionary security measures because it fears mounting tensions over the test could escalate to war.

Russia also called the North Korean ambassador to the foreign ministry and told him Moscow has “serious concern” over this week’s test, the ministry said.

US Secretary of State Hillary Clinton reaffirmed US commitments to allies Japan and South Korea, said North Korea was behaving in a “provocative and belligerent manner” toward its neighbors, and that there were consequences to such behaviour.

Both Moscow and Washington said they hoped North Korea would return to the six-party talks aimed at ending its nuclear programme.