The United States Wednesday unveiled new sanctions against North Korea after the sinking of a South Korean warship and said the attack could be the start of more provocations by the communist state.

Secretary of State Hillary Clinton said the measures were designed to pile pressure on Pyongyang and prevent the regime from bankrolling its atomic programme or spreading nuclear arms.

She said the measures were not directed at the North Korean people, "who have suffered too long due to the misguided and malign priorities of their government".

"They are directed at the destabilising, illicit and provocative policies pursued by that government."

Clinton and US Defense Secretary Robert Gates travelled to South Korea in a show of solidarity after the sinking of the ship, which left 46 dead.

The United States and South Korea, citing findings of a multinational investigation, accuse the North of torpedoing the Cheonan near the disputed Yellow Sea border in March -- a charge it denies.

"There has been some indication over the last number of months, that as the succession process gets under way in the North, that there might be provocations particularly since the sinking of the Cheonan," Gates said.

"So I think it is something that we have to look at very closely, we have to keep in mind and be very vigilant."

Ailing leader Kim Jong-Il, 68, is widely reported to be preparing to name his youngest son as eventual successor.

The top US military officer, Admiral Mike Mullen, also voiced concern about the regime's recent actions, calling Kim "unpredictable" during a visit to some of the 28,500 American troops stationed in the South.

The sinking has sharply raised tensions on the peninsula, 60 years on from the start of the 1950-53 Korean War.

Gates and his South Korean counterpart Kim Tae-Young on Tuesday announced a major joint naval exercise starting Sunday to send a "clear message" to the North.

The South's defence ministry said it would be the first in a series of about 10 joint naval drills in coming months. Gates defended the exercises as important to show "determination not to be intimidated".

And the White House said the exercises would be "defensive in nature" and intended as a show of support in the face of "aggression" from North Korea.

But China repeated its concerns over the wargames, saying the large-scale exercises could aggravate tensions in the region.

Clinton and Gates earlier Wednesday held talks with the South's Foreign Minister Yu Myung-Hwan and Defence Minister Kim, the first so-called "two plus two" meeting in Seoul.

In a joint statement they warned the North of "serious consequences" for future aggression and urged the secretive state to admit it torpedoed the Cheonan.

The US sanctions are aimed at stopping the cash-strapped North from selling nuclear weapons or related material, as well as blocking money laundering and other illicit activities.

Clinton plans to meet with China's Foreign Minister Yang Jiechi on Thursday during a regional meeting in Vietnam.

"We will be consulting with China (on) what we think (are) additional steps that it can take," State Department spokesman Philip Crowley told reporters.

"The secretary will have a chance to explain to Foreign Minister Yang what we have in mind here and the way forward not only on issues related to North Korea but also issues related to Iran," Crowley said.

The punitive measures would strengthen enforcement of UN Security Council resolutions passed after the North's nuclear and missile tests, Clinton said.

She vowed new efforts with other governments to shut down North Korean firms engaged in banned activities and to prevent banks abroad from handling illicit transactions.

By targeting luxury items such as cigarettes, liquor and exotic foods coveted by the regime leadership, as well as singling out influential figures and state organisations, the Obama administration hopes to drive home the price to be paid for international isolation and defiance.

The sanctions would also seek to crack down on North Korean diplomats exploiting their status for "illicit" activities, Clinton said.

She said there was no plan to return to six-party disarmament talks with the North any time soon. Pyongyang would first need to acknowledge its role in the sinking of the Cheonan, commit to scrapping its atomic weapons and halt "belligerent actions", she said.