"We haven't seen any (movement of troops)," the official, who spoke on condition of anonymity, told AFP.

But he added that tracking North Korea was always difficult as it remained "one of the most closed societies in the world."

Seoul's defense ministry earlier said air and ground forces were keeping a closer watch on the land and sea border with the communist North after Pyongyang announced it was abandoning the armistice signed to end the Korean War in 1953.

Tensions have mounted since the North on Monday tested a nuclear bomb and than test-fired five short-range missiles.

Raising the "Watch Conditions" alert level for US and South Korean forces meant stepped up intelligence and surveillance efforts but no redeployment of American troops on the Korean peninsula, the defense official said.

"This is a prudent step ... but it doesn't involve movement of forces," he said.

A spokesman for the Pentagon referred reporters' questions about North Korea to the State Department, saying the US administration was focused on diplomacy.

"This is something that's being handled through diplomatic means at this point," spokesman Bryan Whitman told reporters.

Asked about whether North Korea may have resumed work at the Yongbyon reprocessing plant, Whitman said: "With respect to North Korea and their activities, this is something that the United States government is clearly monitoring."

Monday's nuclear test was believed to be about four times more powerful than the one detonated in 2006 but technical details of the blast and its implications for North Korea's atomic program remained unclear.

"We don't know what we're dealing with here," said a US defense official.

US government experts were still analyzing particulate matter from the test and were expected to release their findings in a matter of days, officials said.

One expert citing seismic data, Jeffrey Park, director of the Yale Institute for Biospheric Studies, said the bomb was less powerful than North Korea hoped because it was detonated incorrectly.