Tensions are close to boiling point on the Korean peninsula after North Korea scrapped a pact aimed at preventing accidental armed clashes with the South.
This followed around 10,000 people marching in the South's capital of Seoul, to call for attacks on the North in revenge for the alleged torpedoing of a Southern warship.
As well as scrapping the pact, Pyongyang's general staff also warned of an immediate attack if the South's navy violates the disputed Yellow Sea borderline, and repeated threats to shut down a joint business project.
Elsewhere in the Yellow Sea, South Korea's navy staged an anti-submarine exercise in its first show of strength since Seoul publicly accused Pyongyang of torpedoing one of its warships on March 26 with the loss of 46 lives.
In Seoul, an estimated 10,000 protesters shouted "Kill our enemy!" and whacked images of the North's leader Kim Jong-Il with sticks.
Investigators from five countries said last week they found overwhelming evidence that a torpedo attack by a North Korean submarine sank the Cheonan near the border.
The South has announced a series of reprisals including a halt to trade. The North, which denies involvement, has responded with angry rhetoric and an announcement that it is cutting all ties with
Blasting the South's "confrontation maniacs, sycophants and quislings", the North's military said agreements on forestalling accidental conflicts would be declared "completely null and void".
Clashes mark recent history
The border was the scene of deadly clashes in 1999 and 2002 and of a firefight in November last year. The South's military believes the Cheonan was sunk in revenge for the firefight.
The general staff said it was scrapping military safety guarantees for South Koreans crossing the border, and would consider a complete block on access to the Kaesong joint industrial estate in its territory.
It also repeated a threat to attack loudspeakers if Seoul goes ahead with its plan to resume cross-border propaganda broadcasts as part of reprisals.
Some 42,000 North Koreans work in 110 South Korean factories at Kaesong, which was developed as a symbol of reconciliation.
"We are watching whether North Korea will take action to cripple the operations of the Kaesong estate, or will close it," said unification ministry spokeswoman Lee Jong-Joo.
South Korea practices drills
Off the west coast, far away from the border, eight South Korean warships launched a drill to destroy mock North Korean patrol boats and submarines. The ships fired cannon and dropped depth charges.
Officials quoted by Yonhap news agency said South Korea's military and the 28,500 US troops in the South had raised their alert level and stepped up aerial surveillance of the North.
South Korea and the United States have launched a diplomatic drive to punish the North with United Nations Security Council sanctions. China, wielding veto power in the council, has held back so far from condemning its ally the North.
The North says the South faked evidence of its involvement in the sinking to fuel confrontation for domestic political reasons.
It threatens "all-out war" against any punitive moves.
US says China will come aboard
US Secretary of State Hillary Clinton visited Seoul on Wednesday in a show of support and said the world had a duty to respond to the North's attack.
A US diplomat travelling with her told reporters Beijing would carefully move closer to Seoul's position.
A visit Friday to South Korea by Chinese Premier Wen Jiabao could mark the start of the change, the official said.
Russia, which also has a Security Council veto, said it would send experts to Seoul to study the findings of the investigation into the sinking.
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", the Kremlin said in a statement.
Once responsibility was established, "the measures judged necessary and adequate by the international community must be taken", it said.
The South Korean-led investigators said parts of a torpedo recovered from the Yellow Sea matched the specifications of one offered by the North for export.
Pyongyang's official news agency late on Wednesday repeated denials of involvement. It said it would have had nothing to gain from the sinking since it was focusing all efforts on reviving the economy by 2012.
source; AFP, SBS Staff @ SBS News Australia