SEOUL (AFP) – 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 in an attempt to fuel confrontation for domestic political reasons. It threatens "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 territorial waters and airspace.

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

The South's decision to wage "psychological warfare" appears to have sparked particular fury.

It has begun installing loudspeakers along the frontier, and has also resumed FM radio broadcasts to the North. In addition, 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 Kaesong jointly-run 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 Pentagon is already planning joint anti-submarine and other naval exercises with South Korea.

"The United States is also reviewing additional options and authorities to hold North Korea and its leaders accountable," Clinton said without elaborating.

The US 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 two days of 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."