Seoul: South Korea and the US on Monday began joint naval drills amid speculation that North Korea would conduct a nuclear test or missile launch to mark the founding anniversary of its ruling Workers' Party.
The exercise, seen as a show of force against North Korea, is being carried out simultaneously in the East and West seas, also known as the Sea of Japan and Yellow Sea, respectively, Efe news reported.
The US sent the USS Ronald Reagan -- a US nuclear-powered aircraft carrier -- to take part in the exercise, the South Korean Navy said in a statement.
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"The 'Invincible Spirit' exercise is aimed at showing the allied forces' strong commitment to counter relentless provocations by the North and improve their maritime interoperability," the statement added.
As well as the USS Ronald Reagan, the allies plan to mobilise dozens of warships and submarines, including destroyers Ticonderoga-class Aegis missile cruisers, P-3 and P-8 maritime patrol aircraft, Apache helicopters and FA-18C Hornet fighters.
The exercise aims to simulate a strike by North Korean special commandos, assisted by submarines and aircraft, attempting to infiltrate across the sea demarcation lines.
The US currently maintains around 28,500 troops in South Korea.
The joint manoeuvres come at a time of growing tension in the peninsula, amid fears that the North could carry out another nuclear or missile test close to the 71st anniversary of the ruling Workers' Party on Monday.
The Kim Jong Un regime has traditionally run these tests around the most important dates on their calendar, such as its latest nuclear test on September 9, Day of the Foundation of the Republic.
In the last three days, satellite images have revealed an increase in activity at North Korea's Tongchanr-ri missile launch base.