SYDNEY, Australia - In a testy public exchange Friday with South Korean President Roh Moo-hyun, President Bush said the United States would formally end the Korean War only when North Korea halts its nuclear weapons program.

The two leaders met on the sidelines of a 21-nation Pacific Rim summit here, spending much of their roughly one-hour session discussing the international standoff over the communist North's pursuit of atomic arms.

They agreed there had been progress. But then they had a before-the-cameras back-and-forth that was remarkable in the diplomatic world of understatement and subtlety.

Roh pushed Bush to be "clearer" about his position on an official end to the 1950-53 Korean War. The two Koreas were divided by the conflict, which ended in a truce, not a peace treaty, meaning they still remain technically at war.

The leaders' tone remained light, but Bush responded firmly: "I can't make it any more clear, Mr. President. We look forward to the day when we can end the Korean War. That will happen when Kim Jong Il verifiably gets rid of his weapons programs and his weapons."

Also Friday, Bush and Russian President Vladimir Putin held private talks on a Europe-based missile defense system, Iran's suspected nuclear program, climate change, Russia's bid to enter the World Trade Organization and other topics.

"We recognize that we can do better solving problems when we work together," said Bush, who elaborated little on their discussion.

Moscow bitterly opposes a U.S. plan to base an anti-missile radar system in the Czech Republic and interceptor missiles in Poland. Led by Putin, it has reacted forcefully against the idea, saying it would spark a new arms race and a repositioning of its missiles. Putin has proposed instead that Russia and the United States share a Russian-rented radar station in Azerbaijan and that missiles could be deployed at sea or in nations such as Turkey.

At Bush's side, Putin asserted that the leaders had agreed that experts from the two sides should meet again and travel to Azerbaijan. Bush made no comment on this.

Also Friday, Bush had lunch with South East Asian leaders. He delivered a speech to business leaders calling on Asia-Pacific nations to keep up the anti-terror fight, not turn away from the fight in Iraq, lead the way toward a worldwide trade agreement and cooperate on addressing climate change.

In the address, Bush also prodded Russia and China to honor democratic principles and allow more freedoms.

The tense moments with Roh came as the leaders each made statements to reporters after their meeting. Roh concluded his by questioning why Bush hadn't mention the issue of the war's end.

"I might be wrong. I think I did not hear President Bush mention a declaration to end the Korean War just now," Roh said through an interpreter. "Did you say so, President Bush?"

"It's up to Kim Jong Il," Bush said.

Roh pressed on. "If you could be a little bit clearer," he said, prompting nervous laughter from the U.S. delegation and a look of annoyance from Bush.

Under a deal reached in February after years of tortuous negotiations, North Korea agreed to relinquish its nuclear programs, including one that has produced bomb material. In return, Washington agreed to open talks on normalizing relations with the North and explore removing a terrorism designation for Pyongyang, among other inducements. The parties to the agreement include China, Japan, Russia and South Korea as well as the U.S. and North Korea.

North Korea shut down its main nuclear reactor in July. And U.S. officials say Pyongyang also has agreed to disclose its nuclear programs and disable them by the end of this year. But the Bush administration is suspicious of the North, believing it cheated on an earlier nuclear deal by starting a separate program to enrich uranium while freezing a plutonium-based one.

The White House acted quickly to downplay the awkward exchange. National Security Council spokesman Gordon Johndroe, said "there was clearly something lost in translation during the photo op."

Johndroe said the United States and South Korea agree on the steps -- already spelled out in the February agreement -- that Pyongyang must take before there can be a full peace agreement.

Roh and Kim are scheduled to meet soon, and Bush said he appreciated that the South Korea leader would urge Kim to adhere to the terms of the deal. "We both agreed on the positive outlook for the six-party talks," Roh said of Bush.

Bush said in his earlier speech that nations must deploy both military might and democratic ideals to turn the tide against extremists. He said countries across Asia should understand the importance of fighting terrorism, since they have so often been its victims.

"Pressure keeps the terrorists on the run, and when on the run, we're safer," he said. "We must be determined, we must be focused and we must not let up."

The president said the best way to open markets was to achieve a breakthrough in global trade negotiations known in the economic world as the Doha round.

"No single country can make Doha a success, but it is possible for a handful of countries that are unwilling to make the necessary contributions to bring Doha to a halt," he said.

On climate change, Bush acknowledged the fears of some that the United States was trying to construct a successor to the Kyoto Protocol outside of international efforts already under way. The Bush administration does not support any international agreement that does not included developing nations, like China and India, that are big energy guzzlers.

"We agree these issues must be addressed in an integrated way," he said. "We take climate change seriously in America."

Climate control has been designated a top agenda item for this year's APEC meeting.

The high-level discussions at APEC could shape talks at a UN conference in December in Bali, Indonesia, that will start to chart a successor to the Kyoto Protocol, which expires in 2012. The United States never ratified Kyoto, which requires 35 nations to cut emissions 5 percent below 1990 levels by 2012.

The U.S has called for a Sept. 27-28 conference in Washington of the 15 biggest polluters.