Yonhap said police had secured warrants for the arrest of two union leaders involved.

BBC: South Korea police break up Yoosung Hyundai strike

The South Korean news agency Yonhap said that two warships equipped with Aegis defence systems would monitor the situation.

BBC: South Korea 'deploys warships to track North missiles'

The Yonhap news said it was expected to make contact with its ground station at 05:00 on Thursday, at which point its operators will be able to make the final judgement on whether the launch achieved its goals.

BBC: South Korea launches space rocket carrying satellite

More than 3, 000 riot police moved in to disperse protesters at the Yoosung Enterprise factory in Asan, south of Seoul, Korea's Yonhap news agency said.

BBC: South Korea police break up Yoosung Hyundai strike

The fishing boat was scheduled to return to its port of Geojin on the eastern coast Friday morning, the official said, according to Yonhap.

CNN: North Korea seizes South Korean boat

The separate five-stage combat alert level, known as "Defcon, " has not changed and remains at stage 4, South Korean defense spokesman Won Tae-jae said at Thursday's briefing, according to Yonhap.

CNN: South Korean, U.S. forces raise surveillance alert level