Russia summoned North Korea's ambassador Thursday to express alarm that Pyongyang could launch a long-range missile, and the isolated nation's other major ally, China, issued its strongest statement of concern to date over the standoff.

South Korea played down the growing tensions, saying a missile firing was not imminent, although the U.S. national security adviser said launch preparations were "very far along."

In an unusual step, Russia's Foreign Ministry called in North Korean Ambassador Pak Ui Chun to say it was alarmed by reports of the planned launch and warn him of Moscow's opposition to any steps that would destabilize the region.

"In particular, the undesirability was stressed of any actions which could negatively affect regional stability and complicate the search for a settlement to the Korean peninsula's nuclear problem," a ministry statement said.

At a regular briefing in Beijing, Jiang Yu, a Chinese Foreign Ministry official, said, "We are very concerned about the current situation. ... We hope all parties can do more in the interest of regional peace and stability."

CountryWatch: North Korea

Worries over a possible North Korean launch have grown in recent weeks after reports of activity at the country's launch site on its northeastern coast where U.S. officials say a Taepodong-2 missile — believed capable of reaching parts of the United States — is possibly being fueled.

Japan and the United States have issued strong statements of concern and have sent ships and planes to monitor the communist nation.

South Korean Defense Minister Yoon Kwang-ung said that, "It is our judgment that a launch is not imminent."

But if the North fires a missile toward South Korean territories, combined U.S. and South Korean forces will be "ready to intercept it immediately," Yoon told a parliamentary meeting.

Seoul has sat for years in the cross hairs of hundreds of North Korean missiles and artillery, and it fears that increased tension could roil its economy.

When asked about South Korea's assessment, National Security Adviser Stephen Hadley said, "We're watching it very carefully and preparations are very far along."

"So you could, from a capability standpoint, have a launch," Hadley said. "Now what they intend to do — which is what a lot of people are trying to read — of course we don't know. What we hope they will do is give it up and not launch."

A top Pentagon official said that a launch would be "a provocation and a dangerous action" that would lead to the United States imposing "some cost" on North Korea.

Peter Rodman, assistant secretary of defense for international security policy, told a House Armed Services Committee hearing that he did not know if such a launch would happen. If it did, Rodman said the Bush administration would take some, unspecified action.

Washington is weighing responses to a potential test that could include trying to shoot down the missile, U.S. officials have said.

The U.S. has urged China, which sends an unknown amount of food aid to the North and is its No. 1 trade partner, to press the North to back down on its potential missile test. President Bush has praised Beijing for "taking responsibility in dealing with North Korea."

Jiang said China would "continue to make constructive efforts."

The North's test of a long-range missile in 1998 shocked Japan and prompted it to accelerate work with Washington on a joint missile defense system.

The communist nation has been under a self-imposed moratorium on long-range missile tests since 1999, when its relations with the United States were relatively friendly. However, it has since test-fired short-range missiles many times, including two in March.

There are diverging expert opinions on whether fueling would mean a launch was imminent — due to the highly corrosive nature of the fuel — or whether the North could wait a month or more.

A North Korean diplomat said in reported comments Wednesday that the country wanted to engage in talks with Washington over its concerns of a possible missile test. The Bush administration rejected the overture, saying threats aren't the way to seek dialogue.

The U.S. instead called on North Korea to return to six-nation talks on its nuclear program.

U.S. Ambassador to the U.N. John Bolton said he was talking with Security Council members on possible action.

The North agreed at talks in September to abandon its nuclear program in exchange for security guarantees and aid, but no progress has been made on implementing the accord.

North Korea has complained repeatedly in recent weeks about alleged U.S. spy flights, including off the coast where the missile test facility is located.

"The ceaseless illegal intrusion of the planes has created a grave danger of military conflict in the air above the region," the official Korean Central News Agency said.

The U.S. has sent ships off the Korean coast capable of detecting and tracking a missile launch, a Pentagon official said. South Korean aircraft have also been flying reconnaissance over the waters between the Korean Peninsula and Japan, said the military official, who spoke on condition of anonymity because he was not authorized to speak on the subject.

Japan said it, too, had sent naval ships and patrol planes to monitor the developments.

The North has claimed to have a nuclear weapon, but isn't thought to have an advanced design that could be placed on a warhead.

Japanese police were preparing for a "worst-case scenario," including the possibility that parts of a missile could fall on Japan, said Iwao Uruma, commissioner general of the National Police Agency.

About 1,000 people, including army veterans and activists, staged an anti-North Korea rally in Seoul, condemning the missile threat.

The two Koreas remain technically at war since the 1950-53 Korean War ended in a cease-fire, not a peace treaty.