The US is considering further steps to hold North Korea to account for the sinking of a South Korea warship, Defense Secretary Robert Gates says.
He told an Asian security conference in Singapore that inaction would set a "bad precedent", but did not say if the US was considering more sanctions.
Seoul has asked the UN Security Council to take action against the North.
South Korean President Lee Myung-bak has described Pyongyang's denial of sinking the ship as "laughable".
During the conference, Mr Gates also urged China to restore military ties with the US, saying their suspension was damaging security in Asia.
Tensions between the two Koreas have increased since the sinking of the Cheonan near the inter-Korean maritime border on 26 March, in which 46 sailors were killed.
North Korea has consistently denied involvement, but an international inquiry found evidence that a North Korean torpedo had destroyed the warship.
Mr Gates said the act was "part of a larger pattern of provocative and reckless behaviour" by the North.
"North Korea must cease its belligerent behaviour and demonstrate clearly and decisively that it wants to pursue a different path," Mr Gates said.
South Korea has not specified what action it wants the security council to take, but diplomatic efforts are focused on China, the North's closest ally.
Without Beijing's backing, analysts say any international effort to tackle Pyongyang is doomed to failure.
On China's suspension of military ties, Mr Gates said the policy made "little sense".
"There is a real cost to any absence of military-to-military relations," he said.
He added that China's suspension of ties "will not change United States policy toward Taiwan".
The head of the Chinese general staff, who was also present at the conference, responded by saying arms sales by the US to Taiwan went against China's "core" interests.