North Korea has renewed activity at suspected nuclear test sites, including the movement of people, vehicles and material, U.S. intelligence sources told FOX News.
Officials said the activity, which has been observed by intelligence agencies for several weeks, does not indicate that a test is imminent, but does increase the likelihood of a nuclear test.
"At some point, you would think they would test it if they truly think they have the capability," one official said.
A test could be carried out without much forewarning, another official warned, adding that a decision to conduct a test by the closed society would only be done with political ramifications in mind.
The new intelligence comes as Asian nations scrambled to forge a common front against Pyongyang's threatened nuclear test, with South Korea warning of a regional atomic arms race that could upend the balance of power in Northeast Asia.
Diplomatic maneuvering kicked into high gear, as Japan, China and South Korea announced a series of summit meetings over the next week to repair damaged ties and coordinate a strategy on North Korea despite no signs of an imminent test.
The joint effort comes just a day after communist North Korea triggered global alarm by saying it will undertake an unprecedented nuclear test in a step toward building the atomic arsenal it views as a deterrent against any U.S. attack.
Huddling in separate summits, Japanese Prime Minister Shinzo Abe will meet Chinese President Hu Jintao on Sunday and South Korean President Roh Moo-hyun on Monday. Roh will then visit Beijing for talks with Hu and other officials on Oct. 13.
The three countries are key players — along with the United States and Russia — in the long-stalled six-nation talks aimed at convincing Pyongyang to give up its nuclear ambitions in return for badly needed economic aid.
It is the first time the North has publicly announced plans to conduct a nuclear test, although recent reports have said it may be preparing one. Pyongyang says it has nuclear weapons, but it has yet to conduct any known test to prove its claim. North Korea claims to have nuclear weapons, but because there has been no outside confirmation, detonating one would be the first proof of its atomic capabilities.
South Korea's top official on dealings with the North, Unification Minister Lee Jong-seok, said Wednesday there were no signs of an imminent test.
Yet Lee warned there was "a high possibility" North Korea would go ahead with one if "efforts to resume the six-party talks fail."
Japan's Asahi newspaper reported that two Japanese spy satellites focusing on a suspected underground test site had not observed any activities apparently connected to test preparations as of Tuesday. The paper cited unidentified government sources.
Meanwhile, Vice Foreign Minister Yu Myung-hwan warned that any display of Pyongyang's nuclear force could prompt Japan to go nuclear and trigger a regional arms race.
Speaking to lawmakers, Yu said such a North Korean nuclear test "could provide a pretext for Japan's nuclear armament.
"This will prompt countermoves by China or Russia and lead to a change in the balance of power in Northeast Asia," Yu said.
In a worst-case scenario, analysts have speculated, a test could push Japan to seek its own nuclear deterrent, intensifying historical tensions with China and South Korea, both of which suffered under Japanese colonial rule in the early 20th century.
Just last month, a think tank run by former Prime Minister Yasuhiro Nakasone proposed in a policy paper that Japan "consider the nuclear option."
A test could also strain the alliance between the United States and South Korea, which has sought to engage its neighbor.
North Korea has boycotted six-nation nuclear talks for nearly a year, angered by American financial restrictions imposed over the North's alleged illegal activities such as money laundering and counterfeiting.
The cooperative efforts displayed Wednesday by Japan, China and South Korea, however, contrast with the fractured reaction to a series of North Korean missile tests in July. In that incident, China and South Korea accused Japan of overreacting.
North Korea faced a chorus of warnings Wednesday.
China — the North's main ally and key benefactor — appealed on Pyongyang to show calm and restraint, issuing an unusually pointed statement that referred to North Korea by name, instead of its usual appeals for all sides to remain calm.
Meanwhile, Russia's defense minister voiced concern about the environmental consequences in neighboring Russian territory.
"The nuclear tests in North Korea, if they take place, could cause ecological damage in Russia," Sergei Ivanov said on a visit to a Russian air base in Kyrgyzstan.
South Korea's Roh called for a "cool-headed and stern" response, while Foreign Ministry spokesman Choo Kyu-ho said a test could cause Seoul to change its engagement policy toward the communist regime.
South Korea has consistently pursued dialogue with North Korea since their leaders first met in a historic summit in 2000. Seoul is also a main aid provider and on Wednesday, it shipping previously promised flood relief aid, including 6,400 tons of cement, despite the nuclear test threat.
"As North Korea has yet to conduct a nuclear test, it is difficult to immediately halt sending flood relief aid, which is being provided on a humanitarian basis," a ministry official said on condition of anonymity, citing official policy.
Other issues could also splinter a common front against North Korea.
Japan, the top U.S. ally in East Asia, has been the most hard-line against Pyongyang, especially since North Korea fired a test missile over Japan in 1998.
China meanwhile is a nominal North Korean ally and benefactor and sees Pyongyang as a counterbalance to U.S. influence in its own backyard.
"North Korea's next step may be to do nothing at all, other than to sit back and watch the rest of the world argue about what to do next," Ralph Cossa, president of the Honolulu-based Pacific Forum, wrote in a report on the latest threat.
Some experts believe the North has enough fissile material to build at least a half-dozen nuclear bombs, though there are doubts about whether it could deliver them accurately on a warhead.
FOX News' Nick Simone and the Associated Press contributed to this report.