North Korea threatened Wednesday to shut a border crossing and open fire on loudspeakers if South Korea makes good on its vow to blare out propaganda across the frontier in revenge for the sinking of a warship.

US Secretary of State Hillary Clinton flew to Seoul to show Washington's "rock-solid" support for its ally amid the rising tensions, and said the world had a duty to respond to the North's torpedo attack.

After a weeks-long multinational probe into the sinking of a South Korean corvette on March 26, investigators said they found overwhelming evidence that a North Korean submarine was to blame.

The findings into the attack which killed 46 young sailors sparked strong international condemnation of the hardline communist state.

The South Monday announced a package of reprisals, including a halt to most trade and a resumption of the loudspeaker broadcasts suspended six years ago.

It is also mounting a diplomatic drive to punish the North through the United Nations Security Council, although veto-wielding member China, the North's sole major ally, is reluctant to sign up.

The North says the South faked evidence of its involvement in the sinking to fuel confrontation for domestic political reasons. It threatened "all-out war" against any punitive moves.

The regime announced late Tuesday it was breaking all links in protest at Seoul's "smear campaign" and would ban South Korean ships and planes from its waters and airspace.

It said relations would remain severed while conservative President Lee Myung-Bak remains in power in Seoul.

The South has begun installing loudspeakers along the frontier and resumed FM radio broadcasts to the North. It plans to scatter propaganda leaflets across the border.

The campaign aims to "push the daily aggravating inter-Korean relations to the brink of war", the North's military said Wednesday, repeating an earlier threat to open fire.

"If the south side sets up even loudspeakers in the frontline area to resume the broadcasting...the KPA (North Korean army) will take military steps to blow up one by one the moment they appear by firing sighting shots."

The North also threatened to ban South Korean personnel and vehicles from a railway and road leading to the jointly run Kaesong industrial estate just north of the border -- a move which would effectively shut it down.

It ordered eight Seoul government officials on Wednesday to leave the estate and switched off two cross-border communications line, Seoul's unification ministry said.

Clinton warned the North to halt its "provocations and policy of threats and belligerence" against neighbours and backed Seoul's moves to take the attack to the Security Council.

"This was an unacceptable provocation by North Korea and the international community has a responsibility and a duty to respond," she told a news conference.

The chief US diplomat said Washington, which stations 28,500 troops in the South, would consider enhancing its defence posture to deter future attacks.

The United States is considering its own sanctions that would hit the North's finances and money flow, a South Korean official told Yonhap news agency on condition of anonymity.

Clinton arrived in Seoul from talks in Beijing at which she pressed China to take a tougher line with the North. So far it has merely urged restraint on all parties.

Clinton gave no indication China was ready to accept Security Council action, but said she expected it to listen to US and South Korean concerns.

"We expect to be working with China as we move forward in fashioning a response to this provocation by North Korea."

Senior US diplomats said they were devising a multi-pronged strategy to isolate North Korea.

"We're working to get statements out of ASEAN, out of the G8, out of the G20, a number of deliberative bodies to make sure that we bring as much diplomatic leverage as possible," said a diplomat returning from Asia with Clinton, on condition of anonymity.

The Association of Southeast Asian Nations and the European Union meanwhile expressed "deep concern" over the rising tensions and called on both Koreas to exercise restraint.

Russia said it would send a team of experts to South Korea to study the results of the international investigation into the sinking.

Russia sending experts to S.Korea to probe warship sinking
Moscow (AFP) May 26, 2010 - Russia will send a team of experts to South Korea to study the results of an international probe into the sinking of one of Seoul's warships, allegedly by North Korea, the Kremlin said Wednesday.

The weeks-long multinational probe into the sinking of a South Korean corvette on March 26 found overwhelming evidence that a North Korean submarine torpedoed the boat, leaving 46 sailors dead.

The North says the South faked evidence of its involvement and has threatened "all-out war" in response to any punitive moves.

"The Russian president, based on a proposal from the leaders of the Republic of Korea (South Korea), has decided to send to that country a group of highly qualified Russian specialists to study in detail the results of the investigation and the evidence gathered," the Kremlin said.

President Dmitry Medvedev "considers it extremely important to establish the precise reason for the loss of the ship and to reveal accurately who is personally responsible for the events," it said in a statement.

Once responsibility was established, "the measures judged necessary and adequate by the international community must be taken," it said.

Medvedev also reiterated a call for restraint on all sides to prevent a rise in tensions "and to maintain peace, security and stability on the Korean peninsula and the whole region," the statement said.

He made the same plea on Tuesday after he spoke by telephone to his South Korean counterpart Lee Myung-Bak.

Russia has a short land border with North Korea and has diplomatic ties as well as some trade links with the isolated Communist state, a one-time ally of the Soviet Union.

Numerous Western nations and Japan have condemned the attack, seen as one of the worst provocations since the end of the 1950-53 Korean War.

The South Monday announced a package of reprisals, including a halt to most trade.

It also mounted a diplomatic drive to punish the North through the United Nations Security Council, although veto-wielding member China, the North's sole major ally, is reluctant to sign up.

US Secretary of State Hillary Clinton flew to Seoul this week to show Washington's "rock-solid" support for its ally amid the rising tensions, and said the world had a duty to respond to the attack.

As a permanent, veto-holding member of the UN Security Council, Russia would need to offer its support for the United Nations to impose sanctions against Pyongyang over the sinking of the Cheonan, a 1,200-tonne corvette.