LONDON -- President Barack Obama and his South Korean counterpart agreed Thursday on the need for a "stern, united" international response if North Korea goes ahead with a planned rocket launch, according to South Korean officials. Obama pledged to push for "peace and stability" as the standoff intensified.

Before vaulting into global talks on the economic crisis, Obama and South Korean President Lee Myung-bak gathered on the sidelines of the G-20 summit and dealt with the latest flare-up with the North.

North Korea says it will send a communications satellite into orbit on a multistage rocket in the coming days. The U.S., South Korea and Japan call the plan a cover for testing long-range missile technology. Obama told Chinese President Hu Jintao separately on Wednesday that the U.S. would consider such an act provocative and that the U.S. would seek U.N. Security Council punishment in response.

After the Obama-Lee meeting, the South Korean presidential office issued a statement saying that the two leaders had agreed to keep working on a verifiable dismantling of North Korea's nuclear programs.

"They agreed on the need for a stern, united response from the international community if North Korea launches a long-range rocket, and to work together in the course of that," the statement added.

The White House had no immediate details on the meeting between Obama and Lee.

Earlier, Obama said in front of reporters that South Korea is one of "America's closest allies and greatest friends" and he lauded Lee's leadership. Obama said the two would discuss a range of issues, including defense and "peace and stability in the Korean Peninsula."

The G-20 summit brings together the world's richest and developing economies. Leaders hope to approve language vowing tough, coordinated rules for financial markets, plus efforts to spark global recovery, while avoiding costly trade disputes.

Obama, his helicopter grounded by the fog, arrived by car at the ExCel center in the city's Docklands district for the Group of 20 leaders summit. The summit's host, British Prime Minister Gordon Brown, gave Obama another warm greeting following their upbeat visit and news conference Wednesday.

Making his first splash abroad as president, Obama says the summit will reflect "enormous consensus" on how to grapple with the world's gravest economic crisis since World War II.

Police in London said more than 80 people were arrested in sometimes violent clashes with protesters who vandalized property in the city's financial district ahead of the summit.

Meanwhile, the world leaders sought to clear up divisions over how far to go with tougher financial regulation.

Obama also has meetings scheduled with Indian Prime Minister Manmohan Singh and Saudi Arabia's King Abdullah. He will likely reassure Singh about plans to boost aid to India's rival, Pakistan. With the Saudi leader, oil prices and Mideast peace efforts are on the agenda, with perhaps a delicate question about the king's recent shake-up in succession plans.

On Wednesday, Obama had his introduction to British royalty with an audience at Buckingham Palace. He and his wife, Michelle, were presented to Queen Elizabeth II and her husband, the Duke of Edinburgh.

Most noteworthy about the meeting may have been a gift Obama gave the queen: an iPod loaded with classic show tunes, including several from "Camelot," which was based on the King Arthur legend, and "My Fair Lady," which was set in London.

Afterward, the queen posed with all the G-20 leaders for a summit class picture.