A nuclear-powered US aircraft carrier will join a major anti-submarine naval exercise with South Korea next week in a show of defiance against communist North Korea, sources said Wednesday.

South Korea, which accuses Pyongyang of torpedoing its Cheonan warship in March with the loss of 46 lives, is also mounting a diplomatic drive at the United Nations against the North but indicated it would not seek new sanctions.

Tensions have soared since investigators concluded last month that a North Korean submarine sank the warship near the disputed Yellow Sea border, prompting South Korea to announce a series of reprisals against the North.

The hardline state has furiously denied involvement and has responded to the reprisals, which include a trade suspension, with threats of war.

On Wednesday North Korean border guards were seen wearing combat helmets, instead of their usual caps, while on duty at the border village of Panmunjom.

"They started wearing metal helmets last Thursday for the first time this year," said Kim Yong-Kyu, a spokesman for the US military in South Korea.

The nuclear-powered USS George Washington will leave its base in the Japanese port of Yokosuka on or around Saturday and arrive in the Yellow Sea early next week, Yonhap news agency said.

Major newspapers carried similar reports. Kim, the military spokesman, said US and South Korean ships would stage a joint exercise but declined to give details.

Sources said the enormous Nimitz-class US carrier was preparing to join the drill.

Yonhap quoted one military source as saying the joint drill was initially planned for late June or early July, but was brought forward to send a "strong signal and show off a firm defence posture".

The George Washington will head a US battle fleet comprising some 10 ships including an Aegis destroyer and nuclear-powered submarines, it said.

South Korea will deploy a 4,500-ton destroyer, a submarine and F-15K fighter jets.

South Korea wants the UN Security Council to take up the Cheonan sinking.

But Vice Foreign Minister Chun Yung-Woo, in Washington for talks with US officials, said Seoul may not push for additional UN sanctions.

"You don't have to think that any Security Council action is for imposing new sanctions," Yonhap quoted him as saying Tuesday after meeting Deputy Secretary of State James Steinberg.

"We expect the Security Council to send a political, symbolic and moral message that such acts as the Cheonan incident cannot be tolerated and that North Korea should be held accountable and should not repeat this kind of military provocation."

The impoverished North is already subject to a range of tough sanctions designed to curb its nuclear and missile development.

Foreign Minister Yu Myung-Hwan said Washington was looking for new ways to dry up the North's hard currency sources, as a way to deprive it of cash for components for nuclear weapons and conventional missiles.

"While the United States is applying existing sanctions thoroughly, it is also paying close attention to the North's production of fake notes, fake cigarettes and drug trafficking," he told the Chosun Ilbo newspaper.

South Korea estimates that its own reprisals will cost the North up to 300 million dollars a year.

UN Security Council action needs backing from veto-wielding members Russia and China, which have traditionally been close to Pyongyang.

The South has briefed Russian naval experts who arrived Monday to review the findings of the Cheonan probe. China, North Korea's economic lifeline, has also been invited to send a team but has not publicly responded.

South Korea on Wednesday dispatched its top nuclear negotiator Wi Sung-Lac to talks in Moscow.

The sinking was expected to have an impact on local elections being held across South Korea Wednesday.

Analysts predict a strong showing for President Lee Myung-Bak's ruling party since cross-border threats at election time -- known as the "North wind" -- tend to cause voters to rally to the conservatives.