A nuclear-powered US aircraft carrier arrived Saturday in South Korea for a three-way exercise involving Japan as they step up military training to cope with North Korean threats, which have escalated following a security pact with Russia.

The arrival of the USS Theodore Roosevelt strike group in Busan came a day after South Korea summoned the Russian ambassador to protest a major deal between Russian President Vladimir Putin and North Korean leader Kim Jong Un this week.

The agreement pledges mutual defense assistance in the event of war. South Korea says the deal poses a threat to its security and warned that it could consider sending arms to Ukraine to help fight off the Russian invasion as a response — a move that would surely ruin its relations with Moscow.

Following a meeting between their defense chiefs in Singapore earlier in June, the United States, South Korea and Japan announced the Freedom Edge drills.

The Roosevelt strike group will participate in the exercise that is expected to start within June. South Korea’s military didn’t immediately confirm specific details of the training.

Rear Admiral. Christopher Alexander, commander of Carrier Strike Group Nine, said the exercise is aimed at sharpening tactical proficiency of ships and improving interoperability between the countries' navies "to ensure we are ready to respond to any crisis and contingency.”

South Korea’s navy said in a statement that the arrival of the carrier demonstrates the strong defense posture of the allies and “stern willingness to respond to advancing North Korean threats.” The carrier’s visit comes seven months after another US aircraft carrier, the USS Carl Vinson, came to South Korea in a show of strength against the North.

The Roosevelt strike group also participated in a three-way exercise with South Korean and Japanese naval forces in April in the disputed East China Sea, where worries about China’s territorial claims are rising.

In the face of growing North Korean threats, the United States, South Korea and Japan have expanded their combined training and boosted the visibility of strategic US military assets in the region, seeking to intimidate the North. The United States and South Korea have also been updating their nuclear deterrence strategies, with Seoul seeking stronger assurances that Washington would swiftly and decisively use its nuclear capabilities to defend its ally from a North Korean nuclear attack. (AP)