The United States has sent an aircraft carrier to South Korea for joint military drills in a show of strength after Vladimir Putin and Kim Jong-un met in Pyongyang.

The announcement came a day after South Korea summoned the Russian ambassador to Seoul to protest against a defence deal signed by Putin and the North Korean leader this week, which included a pledge to come to each other’s aid if attacked.

“The US navy’s aircraft carrier Theodore Roosevelt... arrived at the Busan Naval Base on the morning of June 22,” the South Korean navy said.

Its arrival “demonstrates the strong combined defence posture of the South Korea-US alliance and their firm resolve to respond to the escalating threats from North Korea”, it added.

The carrier’s visit comes about seven months after another US aircraft carrier, the USS Carl Vinson, came to the South in a show of strength against Pyongyang.

The USS Theodore Roosevelt is expected to participate in joint exercises with South Korea and Japan this month. Pyongyang has always decried similar drills as rehearsals for an invasion.

The US, South Korea and Japan have expanded their joint training exercises and heightened the visibility of strategic US military assets in the region to deter the North, which has declared itself an “irreversible” nuclear weapons power.

The carrier arrived a day after Seoul said it had fired warning shots when North Korean soldiers crossed the border in the third such incursion this month.

North Korean soldiers have been engaged in activities such as laying more landmines, reinforcing tactical roads and adding what seemed to be anti-tank barriers near the border, according to the South Korean military.

The two Koreas have also been locked in a “balloon war”, with an activist in the South confirming that he had floated more balloons carrying propaganda to the north.

Pyongyang has sent more than a thousand balloons carrying trash southward, and Kim’s powerful sister Kim Yo-jong has warned that her country is likely to retaliate.