Adm. Robert Willard’s comments 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.”

“I think the prospect of any next provocation … would be a very serious matter for the international community, for (South Korea) and for the alliance as a whole,” he said. He did not comment on the likelihood of such a test.

The head of the U.S. Pacific Command was speaking at a joint news conference in Seoul with Gen. Han Min-koo, chief of South Korea’s Joint Chiefs of Staff. Han said South Korea and the U.S. are keeping an eye on North Korea and are working to deter any planned test.

South Korea’s unification minister, 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, Unification Minister Hyun In-taek told lawmakers, 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.

Recently, however, North Korea has made a series of conciliatory gestures such as releasing South Korean and American detainees, proposing a resumption of stalled joint projects with South Korea, and indicating it is ready to talk again about its nuclear program.

The U.S. stations 28,500 troops in South Korea in a legacy of the Korean War.