SINGAPORE — In a clear challenge to China, US Defense Secretary Robert Gates said Asian nations cannot stand by in the face of North Korea's alleged sinking of a South Korean warship.The sinking, which killed 46 South Korean sailors in March, was part of a reckless pattern of aggression by North Korea, Gates charged Saturday."The question people have to contemplate is, what are the consequences for a North Korea of an unprovoked attack on a neighbor? For nothing to happen would be a very bad precedent here in Asia," Gates said, addressing an international security summit.He did not mention China's financial and diplomatic support for North Korea but said "the nations of this region share the task of addressing these dangerous provocations."The United States and South Korea want China to approve a new international condemnation or punishment of the North. South Korea took its case to the UN Security Council on Friday. China is one of five veto-holding members of the council.China is the communist North's closest ally and largest patron, giving it economic and political pull over an otherwise reclusive and antagonistic government. The United States and South Korea want China to use that clout to rein in the North Koreans.In a tense exchange during the defense conference in Singapore, Gates dismissed suggestions by a Chinese general that Washington was being hypocritical in criticizing North Korea but not Israel for its commando raid on an aid flotilla in the Mediterranean Sea this past week."There is a wide gap in the US attitude and policy to the two instances," said Maj. Gen. Zhu Chenghu of China's National Defense University. He did not endorse the conclusion of a US-backed international investigation that the North sank the warship Cheonan with a torpedo.Gates said the attack on the warship was conducted "without any warning." Israel had issued several warnings to the flotilla not to enter its territorial waters, he said."I won't make judgment on responsibility or fault" in the Mediterranean incident, Gates said, adding that he favors an international investigation to determine responsibility."But I think there is no comparison whatsoever between what happened in the eastern Mediterranean and what happened to the Cheonan," he said.He denied that the sinking revealed holes in the security the large US military presence in Asia is supposed to provide for allies such as South Korea."What it demonstrates is that a surprise and unprovoked attack is very difficult to defend against," Gates said.He also said, without elaboration, that the US is considering "additional options to hold North Korea accountable."The United States and South Korea have already said they plan joint military exercises in response to the Cheonan's sinking, although Gates has said those exercises would probably wait until the Security Council looks at the case.Beyond the show of force and solidarity from those planned exercises, options are limited. Nearly any response could provoke the North further, something Gates and other US officials say they want to avoid.Still, the United States is already beefing up its missile defenses in Asia, and could send additional weaponry or warships to the area.The United States already has applied trade and other sanctions to North Korea. Additional punishment could include the US putting North Korea back on its list of state sponsors of terrorism, although legal opinions differ on whether the Cheonan attack was terrorism.