SEOUL, South Korea -- Showing impatience with Iranian foot-dragging, President Barack Obama said Thursday that the U.S. and its allies are discussing possible new penalties to bring fresh pressure on Iran for defying international attempts to halt its contested nuclear program.

Obama's warning came after Iran rejected a compromise proposal to ship its low-enriched uranium abroad so that it could not be further enriched to make weapons. Talk of fresh sanctions also showed that Obama is preparing for the next phase should Iran fail to meet his year-end deadline for progress in negotiations.

"They have been unable to get to `yes', and so as a consequence, we have begun discussions with our international partners about the importance of having consequences," Obama said at a news conference with South Korean President Lee Myung-bak.

"Our expectation is, is that over the next several weeks we will be developing a package of potential steps that we could take that will indicate our seriousness to Iran."

The tough talk came as Obama wrapped an eight-day, four-nation tour of Asia in which global issues -- nuclear disarmament, climate change, economic recovery -- dominated and goodwill abounded. There also were few new agreements on pending issues.

South Korea, Obama's final stop, was a case in point.

Obama and Lee showed unity on disarming nuclear-armed North Korea and differences over concluding a free-trade agreement stalled by Congress. Obama announced that Stephen Bosworth, his special envoy to North Korea, would make his first trip to Pyongyang on Dec. 8 to test the waters for resuming nuclear disarmament talks.

Lee said Obama endorsed his "grand bargain" for North Korea -- a package of economic assistance and investment in exchange for full nuclear disarmament in a single step rather than the piecemeal approaches that have twice failed over the past two decades. "I think President Lee is exactly right and my administration is taking the same approach," Obama said.

The White House said the trip was largely about showing U.S. re-engagement with a region whose fast-growing economies are reordering global politics but that often felt neglected during the Bush administration and its focus on fighting terrorism. To that end, Obama spoke often of reinvigorating alliances with Japan, his first stop, South Korea and in Southeast Asia, and welcoming a prosperous, confident China as a partner.

"We didn't come halfway across the world for ticker-tape parades," senior Obama adviser David Axelrod told reporters Thursday. "We came here to lay a foundation for progress. We've done that."

Obama vested political capital in salvaging next month's climate change conference in Copenhagen. He urged leaders of Asia-Pacific nations gathered in Singapore to rally around a political agreement that would contain emissions reductions goals for countries to meet that would fall short of a full treaty on global warming. China, the biggest emitter of greenhouse gases ahead of the U.S., signed on to the idea too.

Obama addressed cheering U.S. troops stationed at Osan Air Base outside Seoul on Thursday before the return flight to Washington, and gave this assessment of the trip: a renewed U.S.-Japan alliance, commitments to work on freer trade with Asia-Pacific nations to aid the global economic recovery and a more positive partnership with China "because cooperation between the United States and China will mean a safer, more prosperous world for all of us."

Asked how the trip went, Obama said: "We got a lot of work done." He then boarded the plane headed for home, where he faces continued lobbying to pass a health care bill and more deliberations on how many more troops to send to Afghanistan.

In talking tough about possible sanctions on Iran for its nuclear program, Obama left open the option that diplomacy could still work. "I continue to hold out the prospect that they may decide to walk through this door" and accept the proposal to ship its low-enriched uranium out of the country, Obama said.

A senior administration official later said Obama was purposely vague on more diplomacy so as not to undermine the search for international consensus that remains in an embryonic phase. The official spoke on condition of anonymity to discuss the president's thinking.

Possible sanctions are likely to take months to enact, if the difficulties in crafting this year's U.N. sanctions on North Korea are any indication. China, always reluctant to support sanctions, offered no public assurances that it would agree to punish Iran. As for Russia, whose support also would be vital, White House official Mike McFaul said days ago that the U.S. is "exactly on the same page with the Russians" in exploring diplomacy and consequences.

South Korea gave Obama one of the warmest welcomes during the trip. Crowds lined the motorcade route; some shouted "Obama." After the news conference, Obama and Lee hugged, an unusual gesture in a region noted for its formality.

The only off-note was on the pending free trade agreement, stuck in part because U.S. lawmakers worry it could hurt the struggling American auto industry. Obama said he was committed to completing a deal and that teams from both countries were trying to resolve sticking points.

Lee said the pact was not only economic but strategic -- suggesting an agreement would further cement the U.S.-South Korean alliance. He urged political will to complete it.