The U.S. and its allies may impose new sanctions on Iran and offer economic aid to North Korea in an effort to halt their contested nuclear programs, President Barack Obama said Thursday.

President Barack Obama speaks as South Korean President Lee Myung-bak looks on. Obama discussed strategies to stop Iran and North Korea's nuclear programs. (Kim Jae-hwan/Associated Press)

His warning came after Iran rejected a compromise proposal earlier this week to ship its low-enriched uranium abroad so that it could not be further enriched to make weapons. Talk of new sanctions showed that the U.S. president 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.

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 in early December 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.

U.S. looks to strengthen alliances

Obama wrapped an eight-day, four-nation tour of Asia in which global issues, including climate change, and economic recovery, dominated and goodwill abounded.

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, South Korea and Southeast Asia, while welcoming prosperous China as a partner.

In talking tough about possible sanctions on Iran for its nuclear program, Obama said he expected 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."

Possible sanctions are likely to take months to enact, if the difficulties in crafting this year's UN 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.