SEOUL, South Korea — New international disarmament talks with North Korea are possible only if the North backs off recent aggression against South Korea and demonstrates it is willing to bargain in good faith, U.S. Defense Secretary Robert Gates said Friday.

Gates said diplomacy is worthwhile, starting with direct talks between the North and South. South Korea has rejected new talks for now, reflecting intense anger and impatience over North Korean attacks.

Gates attached no conditions to possible new discussions between the North and South beyond an end to attacks like two in the past year blamed for killing about 50 South Koreans.

He insisted on "concrete steps" by the North for new talks involving the United States.

"When or if North Korea’s actions show cause to believe negotiations could be productive or conducted in good faith, then we could see a return" to dormant six-nation disarmament talks, Gates said.

Those talks include the U.S., China, Japan, Russia and the two Koreas.

Gates made a brief stop in Seoul for crisis talks on North Korea to close a week of military discussions in Asia clouded by the threat of new war on the Korean peninsula.

South Korean Defense Minister Kim Kwan-jin told Gates that his country feels under attack.

South Korea sees recent North Korean aggression as the worst since the close of the Korean War six decades ago.

"Many expect North Korea to conduct more provocation this year," Kim said.

South Korea must answer "from the basis of strength," he added.

Gates and Kim also discussed cooperating militarily to deter aggression by North Korea and urged the North to abandon its nuclear programs and "military adventurism," South Korea’s Defense Ministry said in a statement, without elaborating.

In a meeting with Gates, South Korean President Lee Myung-bak called for cooperation between Seoul and Washington in resolving North Korean issues this year, Lee’s spokeswoman Kim Hee-jung said in a briefing. Lee’s urgency comes amid concerns the North is working to solidify its military and atomic strength as it has set 2012 — the centenary of late North Korea’s founder Kim Il Sung’s birth — as a goal for building a "great, prosperous and powerful country."

The United States fears that the risk of war is rising between U.S. ally South Korea and the heavily militarized and increasingly unpredictable regime in North Korea, which the Pentagon also considers a looming threat to the mainland United States.

North Korea allegedly sank a South Korean warship in March, killing 46 sailors, and shelled front-line Yeonpyeong Island in November, killing four people there. The island sits in waters the North claims.

The U.S. is urging patience but is worried that rising frustration in the South may force its leaders to retaliate if the North attacks again.

Gates was in Tokyo earlier Friday, where he said North Korea was less able to invade South Korea now than it was a decade or more ago but has become a more lethal threat to Asia and the world.

"The character and priorities of the North Korean regime sadly have not changed," Gates said.

"North Korea’s ability to launch another conventional ground invasion is much degraded from even a decade ago, but in other respects it has grown more lethal and more destabilizing," Gates said in an address to students at Keio University.

North Korea’s pursuit of nuclear weapons and missile technology "threaten not just the peninsula, but the Pacific Rim and international stability," Gates said.

Regarding China, Gates said that even as the U.S. military relationship between the two countries improves, at least one area of disagreement continues: "freedom of navigation." That’s a euphemism for the U.S. view that it has the right to sail its ships in waters that China claims as restricted.

Freedom of shipping and commerce have been basic principles for the United States since its founding, Gates pointed out.

He also told students that China’s military sometimes does things without telling the country’s senior political leadership. The communist party has firm control over the military, but "sometimes there are disconnects," Gates said.

"In the larger sense of who controls the Chinese military and who has the ultimate authority there is no doubt in my mind that it is President Hu Jintao and the senior civilian leadership of that country," Gates said, adding the wider U.S. engagement with China he seeks could help cut through the communications problems between China’s military and political spheres.

"I believe we’ve seen instances where specific events take place where the Chinese senior leadership may not have known about them," Gates said.

He mentioned this week’s flight test of the new J-20 stealth fighter as one such example. Gates said Hu did not appear to know about the test until Gates asked him about it.

"But on the whole, I do think this is something that is a worry," Gates said.

He also made the case for the continued presence of tens of thousands of U.S. forces in Japan. U.S. military bases on the southern island of Okinawa have become increasingly unpopular because of noise, crowding and the perception that the U.S. takes Japan for granted.

Most of the 49,000 U.S. forces in Japan are based on Okinawa.

But without those forces, "North Korea’s military provocations could be even more outrageous," Gates said.

"China might behave more assertively toward its neighbors," he added.

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Associated Press writer Kim Kwang-tae in Seoul contributed to this story.