The United States is weighing fresh steps to hold North Korea to account after the sinking of a South Korean warship, US Defense Secretary Robert Gates said on Saturday.

Apart from carrying out military exercises with South Korea and backing action in the UN Security Council, the United States "is assessing additional options to hold North Korea accountable", Gates said in a speech in Singapore.

He did not specify what measures or further sanctions might be under consideration but warned of the risks of inaction after North Korea's alleged torpedo attack on a South Korean naval ship.

He said the sinking of the Cheonan in March, 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.

Gates, speaking at the annual Shangri-La Dialogue on Asia security, called on countries in the region to respond to North Korea's "dangerous provocations".

"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 mainly at China, an ally of North Korea that was slow to react to the incident and has yet to accuse Pyongyang of sinking the South Korean ship.

Gates pledged "full support" to South Korea at a "difficult hour".

He said North Korea faced a choice between remaining an impoverished pariah state or "charting a new path".

"North Korea must cease its belligerent behaviour and demonstrate clearly and decisively that it wants to pursue a different path," he said.

His warning coincided with a South Korean diplomatic push to win backing for a condemnation of the North at the UN Security Council.

Speaking to reporters on Friday, Gates acknowledged that the United States and its Asian allies faced a dilemma in trying to show resolve without ratcheting up tensions and possibly triggering an aggressive response from the hardline regime.

To avoid sparking an over-reaction from the North, Gates suggested joint military exercises might be delayed to give time for diplomacy at the United Nations and that Seoul might seek a 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.

South Korea on Friday formally asked the UN Security Council to respond to the sinking of one of its warships allegedly by a North Korean torpedo, after President President Lee Myung-Bak called the March sinking of the corvette a "military provocation".

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

Seoul's ambassador to the UN, Park In-kook, said he handed a letter to Mexican Ambassador Claude Heller, the council chairman this month, requesting "action by the Security Council commensurate with the gravity of the situation."

Tensions have soared on the peninsula since a multinational probe established that a North Korean torpedo caused the blast that tore apart the Cheonan in the deadliest peace-time incident for Seoul since the end of the Korean War in 1953.

earlier related report
SKorea formally asks UN to respond to ship sinking
United Nations (AFP) June 4, 2010 - South Korea formally asked the UN Security Council Friday to respond to North Korea's alleged sinking of one of its warships, despite its communist neighbor's threat of retaliation.

South Korea's UN Ambassador Park In-kook told reporters he handed a letter to the council's chairman, Mexican Ambassador Claude Heller, requesting action "commensurate with the gravity of the situation."

Seoul wants the 15-member Security Council to "duly consider this matter and respond in a manner appropriate to the gravity of North Korea's military provocation in order to deter recurrence of any further provocation by North Korea," the envoy said.

Heller, who is chairing the Security Council this month, said he would begin consultations with other council members in order to provide an "appropriate answer" to the request.

Heller's spokesman, Marco Morales, later said the letter was indeed circulated to other members and some indicated "they need a bit more time to decide when to meet and work out details of where we go from here."

In his letter, Park described North Korea's "armed attack" against the Cheonan corvette as "a flagrant violation" of the UN Charter, the 1953 Korean armistice accord and a 1992 bilateral agreement on reconciliation, non-aggression and cooperation.

"As such the attack constitutes a threat to the peace and security on the Korean peninsula and beyond," he added.

South Korea has announced a series of reprisals, including cutting off trade with its Stalinist neighbor.

Pyongyang has denied any role in the sinking and has responded to the reprisals with threats of war, sending regional tensions soaring.

South Korean President Lee Myung-Bak earlier called the March sinking of the corvette a "military provocation" and dismissed Pyongyang's denials of involvement as "laughable." He did not mention sanctions.

Tensions have soared on the peninsula in the wake of the deadliest peace-time incident for Seoul since the end of the Korean War in 1953.

South Korean Vice Foreign Minister Chun Young-woo discussed the sinking with several council ambassadors and UN chief Ban Ki-moon, a fellow South Korean, earlier this week.

Seoul can count on full support from the United States, Japan and other Western powers. But in order to secure adoption of the censure resolution, it must also enlist support from veto-wielding council members Russia and China, which have traditionally been close to Pyongyang.

Russia, which has said it needs "100 percent proof" of the North's involvement, sent a team of naval experts to South Korea earlier this week to review findings of the multinational probe and visit the site of the sinking.

The South Koreans have also asked China to send its own experts but Beijing has not responded, according to media reports.

At a three-way weekend summit, China's Premier Wen Jiabao resisted pressure from Japanese and South Korean leaders to publicly support the UN move or to condemn the North, calling instead for efforts to ease regional tensions.

At a security forum in Singapore, Lee warned that while "nobody wants a war," South Korea and its US ally "will respond" if needed.

US Defense Secretary Robert Gates said planned joint US-South Korean military exercises might be put off to allow time for Seoul to secure diplomatic support at the UN Security Council against North Korea.

"I think there's a sequencing involved in this and it may be there's a desire first to see what can be accomplished at the UN and then think about next steps beyond that," Gates told reporters in Singapore.

On Thursday, a North Korean diplomat warned that cross-border tensions were running so high that war might break out "at any moment."

The warship's sinking has stalled efforts to try to revive six-nation talks to rid North Korea of nuclear weapons, on ice since Pyongyang walked out in April last year.