SYDNEY – In a testy public exchange today with President Roh of South Korea President Bush said America would consider the Korean War formally ended 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.

Mr. Roh pushed Mr. 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 Mr. 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 today, Mr. Bush and President Putin of Russia 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,” Bush said, elaborating little on their discussion.

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

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

Also today, Mr. 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. He also said they should lead the way toward a worldwide trade agreement and cooperate on addressing climate change.

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

The tense moments with Mr. Roh came as the leaders each made statements to reporters after their meeting. Mr. Roh concluded his by questioning why Mr. Bush hadn’t mentioned 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,” Mr. Roh said through an interpreter. “Did you say so, President Bush?”

“It’s up to Kim Jong Il,” Mr. Bush said.

Mr. Roh pressed on. “If you could be a little bit clearer,” he said, prompting nervous laughter from the American delegation and a look of annoyance from Mr. 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 exploring the removal of a terrorist state designation for Pyongyang, among other inducements. The parties to the agreement include China, Japan, Russia, and South Korea as well as America and North Korea.

North Korea shut down its main nuclear reactor in July. And American 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. A National Security Council spokesman, Gordon Johndroe, said “there was clearly something lost in translation during the photo op.”

Mr. Johndroe said America 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.

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

Mr. 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, Mr. Bush acknowledged the fears of some that America 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 include 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 U.N. conference in December in Bali, Indonesia, that will start to chart a successor to the Kyoto Protocol, which expires in 2012. America never ratified Kyoto, which requires 35 nations to cut emissions 5% below 1990 levels by 2012.

America has called for a September 27-28 conference in Washington of the 15 biggest polluters.