Gen. Han Min-koo, centre, chairman of South Korean Joint Chiefs of Staff, and U.S. navy Admiral Robert Willard, commander of the U.S. Pacific Fleet, inspect an honour guard in Seoul on Monday. (Ahn Young-joon/Associated Press)

The U.S. Pacific commander has warned that if North Korea carries out a third nuclear test, it would seriously undermine international and regional security.

Admiral Robert Willard's comments in Seoul on Friday were prompted by a South Korean newspaper report that said a U.S. spy satellite detected activity at the North's main nuclear test site and that a detonation could occur in three months. South Korean officials played down the report, saying the activity didn't seem unusual.

Responding to questions about the report, Willard told reporters that North Korea's nuclear capabilities pose a grave threat to the region and that another atomic bomb test — which would be the country's third — would be a "very serious matter," but he did not comment on the likelihood of such a test.

South Korea's Unification Minister Hyun In-taek, who is responsible for relations with North Korea, separately said Friday that it is unlikely that the North will carry out a nuclear test soon.

"We cannot completely rule out the possibility so we are closely monitoring" the North, he told legislators, according to his office. "Still, the chances of North Korea doing so are rather low at present."

North Korea carried out its first nuclear test in 2006 and a second, more powerful one last year, drawing international sanctions each time.

Just before the second test, North Korea walked out of talks aimed at ending its nuclear program.

Tensions between the Koreas — which are still technically at war because the 1950-53 Korean War ended with a truce, not a peace treaty — have been high in recent months following the sinking of a South Korean warship that killed 46 sailors. Seoul blamed the sinking on North Korea, which denied involvement.