A U.S. nuclear-powered submarine arrived in South Korea on Monday, only days after the first U.S. nuclear armed submarine made port in the country in four decades, as the two allies seek to boost American strategic assets to deter North Korea.

The USS Annapolis entered a naval base in South Korea’ southern island of Jeju, to load military supplies while on an unspecified operational mission, the South Korean navy said.

“The two countries’ navies plan to strengthen the combined defence posture with the arrival of the USS Annapolis, and conduct exchange activities to commemorate the 70th anniversary of the alliance,” it said in a statement.

The USS Kentucky, a U.S. ballistic missile submarine, made port in South Korea last Tuesday. It was the first visit since the 1980s of a U.S. nuclear-armed ballistic missile submarine (SSBN) and coincided with the launch of talks between the U.S. and South Korea to coordinate responses in the event of a nuclear war with North Korea.

North Korea launched two ballistic missiles on Wednesday hours after the USS Kentucky’s visit, and again fired several cruise missiles on Saturday.

The USS Annapolis is not nuclear-armed unlike the USS Kentucky and specialises in anti-ship and anti-submarine warfare, and joined trilateral anti-submarine exercises with South Korea and Japan last September in international waters off the Korean peninsula.