"Rhetoric is not enough at this point," Gates said at a roundtable with reporters, according to a transcript released by the Pentagon. "I think there need to be concrete actions by the North to demonstrate that they're truly serious about negotiation and engagement at this point. They could have a moratorium on missile testing, a moratorium on nuclear testing. There are several areas where they could take concrete actions."

Gates was discussing a barrage of peace overtures from North Korea in recent weeks after tensions heightened last year to the highest level since the Korean War with the shelling of a South Korean front-line island and the torpedoeing of a South Korean warship, killing 50 people, including two civilians.

In its most recent proposal for unconditional inter-Korean dialogue, Pyongyang Monday called for a meeting of working-level officials later this month to prepare for possible ministerial-level talks.

Suspicious of the North's history of creating tensions to win economic aid, South Korea proposed that the sides hold talks to discuss the North's shelling of Yeonpyeong Island and the torpedoeing of the Cheonan.

Seoul, Washington and Tokyo insist that Pyongyang apologize for the provocations before any resumption of bilateral or multilateral talks.

China, the North's staunchest ally, has called for an early, unconditional resumption of the six-party talks that also involve the two Koreas, the U.S., Japan and Russia.

"We consider this a situation of real concern and we think there is some urgency to proceeding down the track of negotiations and engagement, but we don't want to see the situation that we've seen so many times before, which is the North Koreans engage in a provocation and then everybody scrambles diplomatically to try and put Humpty-Dumpty back together again," Gates said. "We would like to see are some concrete actions by North Korea that show that they're serious about moving to a negotiation and an engagement track."

The chief U.S. defense official said he had discussed North Korea with Hu.

"We spent some time on North Korea and the importance of some concrete measures on the part of the North Koreans to demonstrate they're serious about proceeding with negotiations and exchanges,"

he said. "The U.S. government recognizes and appreciates the constructive role that the Chinese have played over the last several months in dampening tensions on the Korean Peninsula. All of the evidence that I've seen suggests that the Chinese used their influence with Pyongyang to be restrained in response to any South Korean exercise activity."

North Korea will be high on the agenda during the upcoming summit between U.S. President Barack Obama and Hu in Washington on Jan. 19. Other mutual and global issues include the Chinese currency yuan's revaluation and Iran's nuclear ambitions.

Gates, who flew to Beijing Sunday to mend military ties, strained since early last year due to the Obama administration's plans to sell more than US$6 billion in arms to Taiwan, also expressed concerns over North Korea's missile and nuclear capabilities.

"The first is, with the North Koreans' continuing development of nuclear weapons, and their development of intercontinental ballistic missiles, North Korea is becoming a direct threat to the United States, and we have to take that into account," he said. "I don't think it's an immediate threat, no. But on the other hand, I don't think it's a five-year threat. I think that North Korea will have developed an intercontinental ballistic missile within that timeframe, not that they will have huge numbers or anything like that, but I believe they will have a very limited capability."

North Korea, which detonated nuclear devices in 2006 and 2009, is believed to be behind nuclear and missile proliferation in Iran, Syria, Pakistan and several other countries, as arms sales are considered one of its major sources of revenue. North Korea has for years been under U.N. economic sanctions for its nuclear and missile tests.

Online whistleblower WikiLeaks recently revealed a U.S.

diplomatic cable in which U.S. officials insisted that North Korea had sent Iran 19 advanced missiles that "could clear a path toward the development of long-range missiles" with the capability to hit Western Europe.

North Korea also revealed in November a uranium enrichment plant that could serve as a second way of producing nuclear bombs, aside from its existing plutonium program, despite Pyongyang's claims it is producing fuel for power generation.

David Albright, president of the Institute for Science and International Security, has said that North Korea could have developed nuclear warheads small enough to be mounted on ballistic missiles with the help of China or Pakistan.

Albright noted the seizure of a computer in Switzerland in 2007 that contained a modern nuclear bomb design from the network of A.Q. Khan, a Pakistani nuclear scientist suspected of having provided uranium technology to North Korea in exchange for missile technology.

"It could have transferred from Khan to North Korea, and it could have been China," he said.

China is under intense international pressure to persuade North Korea to refrain from heightening tensions and pursuing nuclear weapons programs.

China, the major provider of food, oil and other necessities to its isolated, impoverished communist ally, has been reluctant to sanction North Korea as any instability could result in a massive influx of North Korean refugees across their shared border or a unified Korean Peninsula under South Korean and U.S. control.

(Yonhap News)