Naval forces of the Republic of Korea, the United States and Japan kicked off a joint drill in waters off the ROK's southern island of Jeju on Thursday, following tensions over Pyongyang's controversial satellite launch in April.
The two-day drill, involving training in search and rescue operations and maritime interdiction operations, will mobilize destroyers, logistics support ships and antisubmarine helicopters, the ROK defense ministry said last week.
Saying the drill was of "a humanitarian nature", the ministry said it was a routine exercise begun by the three nations in 2008. It also said that no live-fire exercises were planned.
The ROK and the US are set to stage a separate, three-day drill from Saturday to Monday in waters off the west coast of the Korean Peninsula.
According to the ministry, the drill will involve antisubmarine exercises, surface ship maneuvers and submarine detection exercises. The US aircraft carrier George Washington is expected to join the drill.
The exercise comes amid rising tensions with the Democratic People's Republic of Korea following its failed satellite launch in April - seen by the US and its allies as an attempted ballistic missile test.
Pyongyang said on Thursday that the three countries' exercise threatens to bring a "new cloud of war" to Northeast Asia.
"DPRK people and the military are watching the trilateral military drill intensely," said the ruling party newspaper Rodong Sinmun, urging the three allies to stop "reckless provocation".
In the drill involving the ROK and US forces on land, the two countries will hold their biggest joint live-fire exercise on Friday. The aim is to showcase their "watertight defense posture and war-fighting capabilities", the ROK defense ministry said earlier this week.
Two thousand troops, F-15K and KF-16 jet fighters and light-attack planes would be among the weaponry deployed. Four US Apache attack helicopters as well as tanks and rocket launchers will fire thousands of rounds during the drill at Pocheon, near the border with the DPRK.
The drill is timed to mark the 62nd anniversary of the start of the Korean War, on June 25, 1950. The conflict sealed the peninsula's division.
The drill indicates that there are concerns about Pyongyang's military power and its possible use of nuclear weapons, said Wang Junsheng, an expert on Eastern Asian studies with the Chinese Academy of Social Sciences.
The trilateral drill shows their Cold War mentality and will make the situation on the Korean Peninsula more complicated and tense, he said.
In the long term, the mechanism of the joint military drill will return Northeastern Asia into a Cold War configuration and hinder the resolution of tension on the Korean Peninsula, he added.
According to Wang, the military drill may help the US president to win re-election.
The close military cooperation is in line with the US strategic shift to the Asia-Pacific region, but the deepening ties between Japan and the ROK is a new phenomenon, said Wang Fan, a professor from China Foreign Affairs University in Beijing.
"Seoul and Tokyo may think that they cannot tackle the regional threats by themselves," he said.
"Viewing the DPRK and China as threats, the US, which has cut its defense budget, seeks to strengthen ties with its Asian allies in order to maintain its influence in the region," he said.
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Xinhua and AFP contributed to this story.