According to Seoul's navy, a US aircraft carrier arrived in South Korea on Saturday as part of cooperative military exercises designed to strengthen defenses against North Korean threats. 

This comes a day after South Korea recalled the Russian ambassador to Seoul to protest the defense agreement that DPRK leader Kim Jong Un and Russian President Vladimir Putin signed in Pyongyang this week. 

The duo agreed to strengthen their nations' relations during Putin's visit to Pyongyang, DPRK state media, KCNA, reported last week, and signed a "comprehensive strategic partnership pact" defense treaty.

The South Korean Navy released a statement saying: "The US Navy's aircraft carrier Theodore Roosevelt... arrived at the Busan Naval Base on the morning of June 22," adding that it "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."

About seven months have passed since the last warship, the USS Carl Vinson visited the South as a show of force against Pyongyang.

The USS Theodore Roosevelt is anticipated to take part in cooperative drills with Japan and South Korea. 

Warning shots

Just yesterday, Seoul's military said that it fired warning shots after DPRK soldiers briefly crossed the heavily fortified border, marking the third such incident this month.

South Korea has previously stated that DPRK has been strengthening the border by constructing tactical roads and increasing the number of landmines, resulting in casualties among its troops due to accidental explosions.

On Thursday morning, "several North Korean soldiers who were working inside the DMZ on the central front line crossed the Military Demarcation Line," Seoul's Joint Chiefs of Staff said.

"After our military's warning broadcasts and warning shots, the North Korean soldiers retreated back northward," they added.

This is the third incident in less than two weeks where South Korean authorities accused DPRK troops of crossing the MDL. However, it was determined that the first incident was unintentional and that the troops who were carrying construction equipment had lost their way on June 9. 

The DMZ has a long history of controversies from both sides and has been a hotspot for nuisances. The two Koreas have sought to creatively disrupt and provoke each other via various methods, including the deployment of loudspeakers that play K-Pop and flying hot balloons that carry garbage and excrement over to the other side.