(UPDATE) SEOUL — A United States aircraft carrier arrived in South Korea on Saturday for joint military drills that aim to better counter North Korean threats, Seoul's navy said.

The announcement came a day after South Korea summoned the Russian ambassador to Seoul to protest a defense deal signed by President Vladimir Putin and North Korean leader Kim Jong Un in Pyongyang earlier 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 in a statement.

Its arrival "demonstrates the strong combined defense 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 around 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 combined drills as rehearsals for an invasion.

The US, South Korea and Japan have expanded their joint training exercises and heightened the visibility of strategic American 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 briefly crossed the heavily fortified border in the third such incursion this month.

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

The two Koreas have also been locked in a tit-for-tat "balloon war," with an activist in the South confirming on Friday that he had floated more balloons carrying propaganda northward.

Pyongyang has already sent more than a thousand balloons carrying trash southward, and Kim's powerful sister Kim Yo Jong warned on Friday that the North was likely to retaliate.