Officials in the United States, South Korea and Japan say there is no sign of a second North Korean nuclear test, rejecting news media reports of a new explosion. The reports rattled nerves in Asia, already tense since Pyongyang declared on Monday it had tested a nuclear device.

Reports of a possible second nuclear test came Wednesday morning from Japan's NHK network, which quoted unidentified officials as saying a seismic tremor had been detected.

However, presidential spokesmen in Washington and in Seoul, and the Japanese prime minister, all said they had no indication of a new test.

North Korean soldiers, background, watch South Korean soldiers, foreground, at the truce village of Panmunjom in the demilitarized zone that separates the two Koreas since the Korean War, north of Seoul, South Korea, Wednesday, Oct. 11, 2006

Monday's apparent nuclear test registered about 3.5 in magnitude. But officials at South Korea's Earthquake Research Center say they detected nothing like that Wednesday. Japanese researchers now say the tremor readings were probably linked to an earthquake in Japan.

Nevertheless, South Korean Deputy Foreign Minister Lee Kyu-hyung says his country is being vigilant. He says even though Wednesday's reports appear false, South Korea is prepared for all possibilities.

The United Nations Security Council is debating a resolution to impose sanctions on Pyongyang for its nuclear program. North Korea says it would consider sanctions an act of war.