U.S. President George W. Bush bluntly told South Korean President Roh Moo-hyun on Friday that the United States would formally end the Korean War only when North Korea halts its nuclear weapons program.

The comment came in a testy exchange between the two leaders on the sidelines of a Pacific Rim summit, after they held talks dominated by the international standoff over the communist North's pursuit of atomic weapons.

In front of television cameras, Bush and Roh agreed there had been progress in trying to resolve the issue, but then had a 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 are still technically at war. The United States also has no peace treaty with the North.

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."

Bush also met Russian President Vladimir Putin on the sidelines of the Asia-Pacific Economic Cooperation forum, and had lunch with a group of Southeast Asian leaders.

Leaders from the 21-member APEC are gathering in Sydney for their annual two-day summit, starting Saturday. Prime Minister Stephen Harper arrived in Sydney Thursday.

In a speech to business leaders meeting alongside APEC, Bush defended his policies in Iraq and urged Asia-Pacific countries to keep up the fight against terrorism. He also urged the region to lead the way toward a worldwide trade agreement and co-operate on tackling global warming.

Bush prodded APEC members Russia and China to honour democratic principles and allow more freedoms.

The awkward moment with Roh came later Friday, when the two leaders each made statements to reporters after their meeting.

"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 an annoyed look from Bush.

Under a deal reached in February after years of negotiations, North Korea agreed to relinquish its nuclear programs. In return, Washington agreed to open talks on normalizing relations with the North and explore removing a terrorism designation for Pyongyang.

North Korea shut down its main nuclear reactor in July, and U.S. officials say Pyongyang has agreed to disable its nuclear programs by the end of this year. But Washington is suspicious the North may renege on the deal.

National Security Council spokesman Gordon Johndroe downplayed the awkward Roh-Bush exchange, saying "there was clearly something lost in translation during the photo op."

Washington and Seoul agree on the steps Pyongyang must take before there can be a full peace agreement, he said.

In his earlier speech, Bush said countries must deploy both military might and democratic ideals to turn the tide against extremists.

"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."

On climate change, the top agenda item at APEC, Bush acknowledged that some countries feared the United States was trying to construct a successor to the UN's Kyoto Protocol outside of international efforts already under way.

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

The U.S has called for a Sept. 27-28 conference in Washington of the 15 biggest polluters. A broader conference will be held at the UN in New York on Sept. 24.

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 Kyoto.

On trade, Bush said the best way to open markets was to achieve a breakthrough in the global negotiations known 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.