Beijing - South Korea said Sunday that it would ask the U.N. Security Council to punish North Korea for its deadly attack on a South Korean warship, a move that could ratchet up pressure on the isolated Stalinist regime and add a new flash point in U.S. relations with China.

South Korean President Lee Myung-bak will make the request in an address to his nation today during which he will detail a package of measures in response to the March 26 torpedoing of the 1,200-ton Cheonan and the killing of 46 sailors, said his spokesman Lee Dong-kwan.

A senior U.S. official, traveling with Secretary of State Hillary Rodham Clinton in China, said the United States would back "all the steps the South Koreans are going to announce tomorrow." In an indication of the seriousness with which the Obama administration views the unfolding drama between the North and the South, home to nearly 29,000 U.S. troops, he added: "We have not faced something like this in decades."

Among other measures that could be pushed by Lee, analysts said, were cuts in South Korean trade with the North, returning North Korea to the U.S. State Department's list of states that sponsor terrorism and tighter U.N. sanctions on Pyongyang. Lee has apparently ruled out military action because he does not want to trigger an all-out war.

The official said that, based on talks over the past two days, Chinese officials have not accepted the results of a South Korean investigation - backed by experts from the United States, Australia, Britain and Sweden - that implicated North Korea in the attack. As such, it is unclear whether Beijing will support Lee's call in the Security Council.

China's reluctance to agree with the report underscores the challenges the United States faces as it seeks to forge closer ties to Beijing. The U.S. official also noted Sunday that China and the United States still do not see eye to eye on the details of planned economic sanctions on Iran for its failure to stop its nuclear-enrichment program. Of specific concern, he said, were disagreements between Beijing and Washington about how investments in Iran's oil and gas sector would be treated. China has committed to investing more than $80 billion in Iran's energy sector; tightened sanctions against Tehran could threaten those investments.

U.S. officials said the Obama administration considers the situation in Northeast Asia and Iran so pressing that on Sunday night in Beijing, Clinton dispensed with the niceties of protocol and got down to a substantive discussion in the middle of a private banquet to welcome the biggest delegation of U.S. officials to Beijing to date. The officials - a band of 200 led by Clinton and Treasury Secretary Timothy Geithner and specializing in fields such as health, energy and the environment, counterterrorism, nuclear proliferation and human rights - are in Beijing for the U.S.-China Strategic and Economic Dialogue.

Officials and analysts said that the attack on the Cheonan seems to be redefining the security equation in Northeast Asia, bolstering the United States, damaging China and concentrating the minds of Japanese officials.

The attack has provided political cover for Japan's government - only the second opposition party to take power in nearly 50 years - to end an eight-month-long feud with the United States and accept a plan to relocate a U.S. Marine base within Okinawa. On Sunday, Japanese Prime Minister Yukio Hatoyama announced that his country would abide by a 14-year-old agreement with the United States to move the Futenma air base in Okinawa to a less populated part of the island. U.S. officials responded cautiously, however, because important details have yet to be ironed out.

Hatoyama's government had campaigned on a platform that rejected the Futenma deal and advocated a more Asia-centric view of Japan's place in the world. But the Cheonan incident reminded them "that this is still a very dangerous neighborhood and that the U.S.-Japan alliance and the basing arrangements that are part of that are critical to Japan's security," the senior U.S. official said.

The attack and its aftermath also threaten China's place in the region and could force it to make an unwanted choice between South Korea and North Korea - two countries that it has handled deftly since normalizing relations with Seoul in 1992. South Korea wants China, which is a permanent member of the Security Council, to back Seoul's call to take the Cheonan issue to the council. So does the United States, the U.S. official said.

But that could risk hurting Pyongyang, and China appears committed to maintaining the North Korean regime above all.

"For China," the U.S. official said, "they are in uncharted waters."

China reacted slowly to the Cheonan's sinking, waiting almost a month before offering South Korea condolences. Then it feted North Korean leader Kim Jong Il in early May, apparently offering him another large package of aid, Asian diplomats said. China's attitude has enraged South Korean officials.

Michael Green, a national security official during George W. Bush's administration, said the Cheonan crisis highlights just how differently China views its security needs than the rest of the players in Northeast Asia. For years, as China worked with the United States, Russia, South Korea and Japan to try to persuade North Korea to give up its nuclear weapons programs, these differences were obscured. But the Cheonan's sinking has changed that.

While the incident is pushing officials in South Korea, Japan and the United States to contain North Korea and even prepare for a future without a North Korean state, Green said, China appears intent on redoubling its efforts to ensure North Korea's stability.

"The Chinese are very negative about the prospect of a democratic, united Korea on their border," Green said. "They want to keep North Korea alive."