The Democratic People's Republic of Korea (DPRK) threatened on Wednesday to block cross-border traffic and blow up any Republic of Korea (ROK) loudspeakers blasting propaganda northward after a six-year hiatus, as tensions soared over the sinking of an ROK warship.

The dramatic deterioration in relations came as US Secretary of State Hillary Rodham Clinton visited Seoul at the end of a three-nation Asian tour that earlier took her to Japan and China.

Clinton on Wednesday voiced unequivocal support for the ROK in dealing with the sinking of the Cheonan, which Seoul claims was torpedoed by the DPRK, calling Pyongyang's "aggression unacceptable."

At a briefing on Premier Wen Jiabao's upcoming visits to the ROK, Japan, Mongolia and Mynmar, Vice-Foreign Minister Zhang Zhijun said Beijing regards the sinking as "extremely complicated".

"China is carefully and prudently studying and examining the information from all sides," Zhang told reporters at a briefing in Beijing.

He said Beijing was following developments and reiterated China's call for all sides to engage in dialogue and avoid hostile actions that could escalate tensions.

In Seoul, Clinton said during a joint press conference with ROK Foreign Minister Yu Myung-hwan that Washington is considering additional options to hold Pyongyang accountable for the sinking of the 1,200-ton corvette. She did not elaborate on what options are on the table at the moment.

"This was an unacceptable provocation by North Korea (DPRK), and the international community has a responsibility and a duty to respond," Clinton told reporters after talks with ROK leaders.

The DPRK flatly denies it caused the sinking and has warned that retaliation would lead to war.

On Tuesday, Pyongyang announced it was cutting relations with the ROK, starting "all-out counterattacks" against the South's psychological warfare operations and barring ROK ships and airliners from passing through its territory.

On Wednesday, it cut off some cross-border communication links and expelled eight ROK government officials from a joint factory park in the DPRK border city of Kaesong, ROK's Unification Ministry said.

The North's military also issued a statement warning it would "totally ban" the passage of ROK personnel and vehicles to an inter-Korean zone in the western coastal area, apparently referring to Kaesong, if the South does not stop its psychological warfare. It did not mention another border crossing on the eastern side of the peninsula, which remained open.

The statement said it would shoot at and "blow up" any loudspeakers the ROK installs at the border. Seoul dismantled such devices six years ago amid warming ties, but resumed radio broadcasts into the North on Monday and said loudspeakers would be reinstalled within weeks.

"The South Korean (ROK) puppet warlike forces would be well advised to act with discretion, bearing deep in mind that such measures of the Korean People's Army will not end in an empty talk," said the statement, carried by the official Korean Central News Agency.

However, the DPRK still allowed ROK workers to cross the border to enter the Kaesong complex on Wednesday, according to the Unification Ministry.

The North's statement on Tuesday did not refer to about 800 ROK company managers and workers at Kaesong. Seoul also excluded the zone - the last remaining major joint reconciliation project - from its retaliatory measures.

The ROK's military said on Wednesday there were no signs of unusual activity by DPRK troops.

The ROK wants to take the DPRK before the UN Security Council over the sinking, and has US support.

The US and ROK are planning two major military exercises off the Korean Peninsula, the White House said. The US has 28,500 troops in the ROK.