The United States yesterday weighed fresh steps to hold North Korea to account over the sinking of a South Korean warship, after Seoul appealedforUN intervention.

Washington" is assessing additional options to hold North Korea accountable", apart from the UN Security Council route and planned military exercises with South Korea, Defence Secretary Robert Gates said in Singapore.

He did not specify what the new measures might be but, in an apparent message to China, warned of the risks of inaction after North Korea's alleged torpedo attack on the ship in March, which killed 46 South Korean sailors.

South Korea on Friday formally asked the UN Security Council to respond to the sinking, after President Lee Myung-Bak called the attack on the Cheonan corvette a "military provocation".

Addressing the same Singapore conference as Gates, Lee dismissed Pyongyang's denials of involvement as "laughable" but he stopped short of calling for specific sanctions on the North.

Gates said the sinking, which an international investigation blamed on Pyongyang, was not an isolated incident but "part of a larger pattern of provocative and reckless behaviour" by the North.

The Pentagon chief, speaking at the annual Shangri-La Dialogue on Asian security, called on countries in the region to respond to North Korea's "dangerousprovocations".

"Inaction would amount to an abdication of our collective responsibility to protect the peace and reinforce stability in Asia," he said.

His remarks appeared aimed at China, an ally of North Korea that was slow to react to the incident and has yet to accuse Pyongyang of sinking the Cheonan.

Gates pledged "full support" to South Korea at a "difficult hour" but avoided talk of any US or allied military response.

The South Korean and US navies were scheduled to hold a joint anti-submarine drill next week, as a show of force against North Korea.

But the exercise has been postponed, South Korean officials said Friday, and Gates suggested a delay would allow time for UN diplomacy to play out.

He also said that Seoul might seek a Security Council letter denouncing Pyongyang instead of a full-fledged UN resolution.

Such a diplomatic strategy is "not a manifestation of a lack of recognition of the nature of the provocation we have seen from North Korea, but may be more addressed to the worry about provoking further instability and further provocations from the North", he said.