The United States has warned North Korea not to conduct a second nuclear test.
The warning was issued by White House spokesman Tony Snow, amid U.S. television reports American spy satellites have detected vehicle movements in the North that may signal preparations for another test.
South Korean officials also say they are aware of signs Pyongyang may be preparing another test.
Snow said it is not unreasonable to expect the North Koreans to carry out another nuclear test, but said such a development would, in his words, "not be a good thing for them." He said North Korea has made no secret of its desire to be provocative.
Earlier (Tuesday), North Korea said sanctions ordered by the United Nations after its nuclear weapons test last week are the same as a declaration of war. It said Pyongyang would take what it calls "merciless" action against any country that infringes on its sovereignty.
A senior American diplomat, Assistant Secretary of State Christopher Hill is in South Korea for talks on how U.S. allies in the region should respond to the North's nuclear test.
Secretary of State Condoleezza Rice has departed for a visit to North Korea's neighbors, Japan, South Korea, China and Russia to talk about the sanctions and how to convince the reclusive country to return to nuclear disarmament talks.
Rice is expected to meet with Japanese, South Korean and Chinese officials to discuss coordinating inspections of cargo going into and out of North Korea.
China said today it would abide by the U.N. sanctions. The Foreign Ministry said the Chinese side has always implemented Security Council resolutions seriously, and in a responsible manner. China is one of North Korea's few allies.