South Korea and China are warning North Korea of consequences if it goes ahead with a third nuclear test.
There is increasing speculation North Korea will attempt to conduct another nuclear test, perhaps within the next one or two weeks.
South Korean foreign ministry spokesman Cho Byung-je warns any such action will violate international sanctions and further isolate the impoverished country.
Cho says, as far as the South Korean government knows, there are no signs North Korea is about to conduct such a test.
Kim Min-suk, a spokesman at South Korea's defense ministry, concurs. Kim explained that the military has no specific indications that a third attempted test is imminent. He said it is difficult to predict what will happen, but South Korean and U.S. combined forces are paying close attention and utilizing all assets to obtain more information.
China's vice foreign minister, Cui Tiankai, said Beijing will oppose any action that could destabilize the region.
Speaking to reporters Wednesday, Cui said no party - clearly a reference to North Korea - should take any action that might escalate tensions. He said maintaining peace and stability on the Korean peninsula and in Northeast Asia is the joint responsibility of all sides, not just China alone.
According to a Chinese newspaper, Beijing is strongly opposed to North Korean nuclear testing and will not be able to shield North Korea from the diplomatic consequences if it goes ahead with another one.
The warning appeared in a Global Times commentary, which is affiliated with China's ruling Communist Party.
South Korea's intelligence agency says it cannot confirm a Kyodo news service report that Russian forces have gone on alert anticipating a North Korean nuclear test within a week.
The report quotes an unnamed security official in the Russian Far East Pacific maritime region.
South Korea this month made available to news organizations satellite surveillance photos showing fresh digging at the site where North Korea attempted two previous nuclear tests.
Traces of radioactive elements were detected in the atmosphere following the 2006 test. But nothing was detected after North Korea's second claimed underground nuclear detonation in 2009. Both of those events followed failed long-range missile tests in North Korea.
A similar multi-stage rocket was launched from a new facility in North Korea on April 13. It exploded over the Yellow Sea two minutes after lift-off.
In the past few weeks, Pyongyang has amplified its bellicose rhetoric directed at Seoul. It has threatened swift military action against the South Korean government. It says South Korea's president, Lee Myung-bak, must be punished for several disrespectful comments about the North since the December death of leader Kim Jong Il.
Analysts are split on how seriously to take the threats. Some contend they are meant primarily for domestic consumption to bolster the legitimacy of new leader Kim Jong Un. Others note that similar warnings have preceded limited military strikes against the South.
While North Korea has artillery capable of hitting the heavily populated South Korean capital, the South and its U.S. ally have weaponry - including fighter and bomber aircraft - capable of striking anywhere in the heavily militarized North.