A top South Korean envoy arrived in China on Tuesday to seek Beijing's support for a UN censure of North Korea over the sinking of a warship, but reaffirmed Seoul would not ask for new sanctions.

There would be "no practical benefits" to any new Security Council sanctions since such measures were already in place, Vice Foreign Minister Chun Yung-Woo told Yonhap news agency before departing for Beijing.

"But the council is the primary organisation responsible for maintaining international peace and stability," Chun said. "In the context of such responsibility, it needs to take measures over the Cheonan incident."

South Korea last week referred the attack to the council, after investigators from five countries concluded last month that a North Korean submarine sank the Cheonan warship in March with the loss of 46 lives.

The UN move sparked a furious response from the North, which denies involvement and has warned that the crisis could trigger all-out war.

Mass rallies were held in two provinces "to slash at the US imperialists and the Lee Myung-Bak group of traitors' anti-DPRK (North Korea) smear campaign", ruling party newspaper Rodong Sinmun said, referring to the South Korean president.

The South can expect support from the United States, Britain and France for its bid to have the world body reprimand the North. China and Russia, the other two veto-wielding council members, have not publicly stated their position.

China so far has not publicly condemned its ally the North and appealed only for restraint. Russia sent naval experts to South Korea to examine the evidence.

Chun, who is in charge of UN affairs, plans to meet his Chinese counterpart Cui Tiankai during his two-day visit to Beijing.

A Chinese foreign ministry spokesman confirmed Chun's arrival but gave no specifics on any planned discussions.

On a visit to Hanoi, US Assistant Secretary of State Andrew Shapiro repeated Washington's firm support for Seoul, and for "concerted international action to send a message to North Korea that this behaviour is unacceptable."

"So we are looking to support our ally South Korea and we are encouraging other nations to do the same," he said.

The UN tightened sanctions on North Korea after its missile launches and second nuclear test last year, curbing its lucrative weapons trade.

The head of South Korea's military intelligence unit said Tuesday the North may follow up its attack on the Cheonan with a cyber-attack to disrupt the Group of 20 summit in Seoul in November.

Major General Bae Deuk-Shik, chief of the Defence Security Command, told a military seminar the North has an army unit of elite hackers.

"Chances are getting higher than at any other time that North Korea may launch a large-scale cyber attack to disrupt the G20 summit," he said.

Intelligence officials blamed the North for cyber attacks that briefly crippled US and South Korean government and commercial websites last July.

Defence Minister Kim Tae-Young also called for swift action against Internet rumourmongering about the sinking. Young Internet users in South Korea have raised doubts about the official probe.

Intelligence officials have said the North has mounted a cyber campaign — using stolen identities of South Korean Internet users — to spread its own claims about the sinking.

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