SEOUL, South Korea - Secretary of State Hillary Rodham Clinton opened a U.S. campaign Friday for international measures to punish North Korea for the sinking of a South Korean warship, but it was unclear how receptive Pyongyang's benefactors in China - or even South Korea would be.

Clinton began an Asian tour making clear that the Obama administration wants the United Nations to take action against North Korea for sinking the patrol boat Cheonan in March, killing 46 South Korean crew members.

South Korea said this week that a multinational investigation had concluded that North Korea fired a torpedo that sank the vessel.

"Let me be clear. This will not be, and cannot be, business as usual," Clinton said in Tokyo at an appearance with her Japanese counterpart, Katsuya Okada. "There must be an international - not just a regional, but an international - response."

Clinton did not suggest specific actions. North Korea already is under a series of U.N. sanctions, but analysts said that additional steps still could make it more difficult for North Korean organizations to do business abroad and could further isolate the country's rulers.

Clinton travels to China this weekend, and it may prove difficult for her to make headway there on the North Korea issue. Beijing considers itself a protector of its isolated neighbor, and is likely to resist punitive new measures. Pushing China on the question may be difficult for Clinton, who also is trying to hold Beijing's support for sanctions against Iran.

With China reluctant, analysts said they expect a struggle within the U.N. Security Council on the question of new measures against North Korea.

After meeting this weekend with Chinese leaders, Clinton is due to travel to Seoul on Monday. South Korean President Lee Myung Bak has threatened "stern" action, while North Korea has vowed "all-out war" as a response to any retaliation.

The sinking of the Cheonan has been called South Korea's 9/11, a national wake-up call that resulted in weeks of televised images of sobbing mothers and grim-faced generals. Officials in Seoul were considering steps including severing economic aid to the North, realigning military forces, and asking the U.N. Security Council to rebuke Pyongyang.

Defense Minister Kim Tae-young said "the government will definitely make sure North Korea pays," and some commentators have called the sinking of the vessel an act of aggression.

But opinion in South Korea is far from unanimous. The opposition, which favors greater engagement with North Korea and is suspicious of the pro-U.S. leanings of the governing conservatives, has been slower to blame North Korea, even in some cases questioning the authenticity of the evidence.

The opposition-leaning Hankyoreh newspaper challenged the premise that a North Korean submarine would have been able to get close enough to the Cheonan undetected, fire on it, and then escape. Even if the North Koreans were responsible, it said, the event shouldn't be used to mask South Korean failures that allowed the attackers to succeed.

And among younger South Koreans born after the war of the 1950s, many view the North less as an evil empire than a colony of misguided cousins with whom they will one day become reunited.

"People just doubt and doubt and doubt," Lee Ji-sook, 22, said of her government's accusation that North Korea launched a surprise submarine attack.

Lee, an urban-planning major at Yongsei University who is spending a year abroad in the United States, said that many in her generation don't believe North Korea poses a threat to the South.

"There is no common consensus on the Cheonan incident," said Kim Yong-hyun, professor of North Korean studies at Dongguk University in Seoul.

Instead, Kim said, many South Koreans believe that the government needs a convenient foil so it can sell its agenda of increased security in June 2 elections for local and regional positions.