South Korea's military said on Friday that it had fired warning shots after North Korean soldiers briefly crossed the demilitarized border, the third such breach announced this month.

It comes days after North Korea and Russia said they had signed a comprehensive security agreement in a move condemned by Seoul.

Also on Friday, German Vice Chancellor Robert Habeck visited the heavily fortified border that cuts through the Korean Peninsula during the first leg of his days-long trip to East Asia. Habeck is also stopping in China.

"It is a very different border to the one we know from the German division," Habeck said during the visit.

What do we know about the latest border incursion?

South Korea's Joint Chiefs of Staff said that North Korean soldiers retreated after the warning shots were fired.

The soldiers breached the demarcation line in the middle of the Demilitarized Zone (DMZ) around 11 a.m. (0200 GMT) on Thursday.

South Korea's military also fired warning shots on Tuesday after dozens of North Korean soldiers breached the demarcation line. A similar incident also occurred on June 9.

Seoul said that both of the previous crossings appeared to have been accidental.

North Korea has been reinforcing the border in recent months, adding tactical roads and laying more landmines.

The DMZ was established by a US-led UN command at the end of the Korean War in 1953. The fighting stopped with an armistice agreement, but a peace treaty was never signed.

South Korea summons Russian ambassador

Also on Friday, South Korea's Foreign Ministry said it had summoned the Russian ambassador in protest of this week's security deal between Russia and North Korea.

North Korea had hosted Russian President Vladimir Putin, where he signed a mutual defense agreement with North Korean leader Kim Jong Un.

Deputy Foreign Minister Kim Hong-kyun said that Russia's military support for North Korea would have a "negative impact" on relations between Seoul and Moscow.

Seoul "strongly urged Russia to immediately stop military cooperation with North Korea and comply with [UN] Security Council resolutions," the foreign ministry said in a statement.

Kim Hong-kyun urged Moscow to "act responsibly," according to the Foreign Ministry.

The Russian ambassador to South Korea said in response that "threats and blackmail" against Moscow over its ties with Pyongyang are unacceptable, Russia's state news agency TASS reported.

Incursions come amid heightened tensions

Relations between Seoul and Pyongyang have been increasingly strained in recent months.

The two neighbors have also been embroiled in a so-called "balloon war." Pyongyang has been sending thousands of trash-filled balloons south, which it says are in response to balloons carrying anti-Pyongyang propaganda sent north by activists.

The last episode on Thursday saw South Korean civilian activists, led by North Korean defector Park Sang-hak, send 20 balloons to the North, carrying some 300,000 propaganda leaflets, 5,000 USB sticks with South Korean pop songs and TV dramas, and $3,000 bills.

The powerful sister of North Korean leader Kim Jong Un issued a seemingly retaliatory threat in response on Friday.

"When you do something you were clearly warned not to do, it's only natural that you will find yourself dealing with something you didn't have to," North Korea's official Korean Central News Agency cited Kim Yo Jong as saying.

Separately, North Korea has already made several weapons transfers to Russia since August, according to South Korean national intelligence.

In response to the deal, South Korea said it would "reconsider" its policy on weapons deliveries to active conflict zones that currently bars it from supplying Ukraine with direct military aid.

South Korea is a major arms exporter and has so far only sold weapons to Kyiv's allies but not directly to Ukraine.

sdi/rmt (AFP, AP, dpa, Reuters)