North Korea threatens to send army into demilitarised border zone
Kim Yo-jong, the sister of the North Korean leader, has "soon" threatened to take action.
The North Korean army has warned that it is ready to enter the demilitarized zone between the two Koreas.
The threat is partly a reaction to defector groups in the south sending propaganda material north.
At the weekend, Kim Yo-jong, sister of North Korean leader Kim Jong-un, said she had ordered the army to prepare for the move.
And the military is now saying that it is ready to "turn the front into a fortress and increase military vigilance".
The tensions between the two countries have been increasing for some time now, because of the cross-border leaflets that are usually sent out by balloons.
The South Korean Department of Defense responded to the renewed threats on Tuesday by saying that it was working with the United States to closely monitor military movements in the north.
What did the north say?
North and South Korea are separated by the so-called demilitarized zone (DMZ) - a buffer along the border that has separated the two countries since the Korean War in the 1950s.
On Tuesday, the North Korean military said it was studying "an action plan" for the army to "move to the demilitarized zones."
The General Staff said he was "on high alert" and ready to implement government decisions "quickly and thoroughly".
The explanation comes after the sister of North Korean leader Kim Jong-un threatened with military action against the south on Saturday.
"I think it's high time to break with the South Korean authorities," said Kim Yo-jong, who holds a leading role in the Northern political hierarchy.
She promised to "take action," said she had directed the military and ended her statement: "Garbage must be thrown in the trash can."
Why is North Korea doing this?
Analysis box by Laura Bicker, Seoul correspondent
South Korea takes these threats seriously.
Intelligence in the DMZ has been strengthened, and President Moon has asked for calm and Pyongyang to avoid escalating tensions.
But how did this point come about about leaflets?
First, North Korea has a point about propaganda against the regime. The South Koreans pledged to prevent the leaflets from being flown across the border as part of the 2018 inter-Korean deal between President Moon and Kim Jong Un.
Second, Pyongyang is angry with Seoul for not questioning the United States' insistence on strict sanctions.
So it is likely that it is something bigger. The timing of the releases and the cautious escalation from interrupting communications to threats from military action appear to have been orchestrated.
North Korea could prepare for a crisis to punish the south and possibly use the tension as a lever for future talks.
The threats themselves are aimed at squeezing President Moon's hard-won profits in 2018.
A total of 20 watchtowers were demolished with the hope of turning the most fortified border in the world into a peace zone.
President Moon said he wanted to create "irreversible peace" on the peninsula. The North Koreans could prove the opposite.
What is the leaflet series?
Last week Pyongyang cut all communications with the South, including a hotline between the leaders of the two nations.
It was said to be upset with North Korean defectors from the south who were sending leaflets across the border.
Groups led by defectors often send balloons across the border and carry leaflets and other items, including groceries, $ 1 bills, radios, and USB sticks with South Korean dramas and news.
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The South Korean government has already tried to stop the groups, arguing that their actions are putting residents near the border at risk.
President Moon Jae-in directly appealed to the North on Monday to return to dialogue and not escalate the situation further.
What is the demilitarized zone?
The Demilitarized Zone (DMZ) was established after the Korean War in 1953 to create a buffer zone between the two countries.
Over the decades, shots have occasionally been fired, North Korean soldiers have escaped, and peace talks have been held to ease tensions. It is even a tourist destination from the south.
The Korean border is one of the most militarized areas in the world
During the relative rapprochement between North Korea and the United States, the DMZ was also the scene of direct handshakes between Kim Jong-un and Donald Trump and Moon Jae-in.
In the past two years, the South Korean government in Seoul has tried to turn the heavily fortified border into a peace zone.
At a summit in Pyongyang in September 2018, a relaxation of the military tensions at the border was agreed between the leaders of the two countries.
So far, despite its name, the zone has been one of the most militarized areas in the world.
International peace talks had raised hopes that tensions could subside
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