North Korea's army has warned it is ready to enter the demilitarised zone dividing the two Koreas.

The threat is partly in response to defector groups in the South sending propaganda material north.

Over the weekend, Kim Yo-jong, the sister of North Korean leader Kim Jong-un, said she'd ordered the army to prepare for the step.

And the military now says it is ready to "turn the front line into a fortress and heighten military vigilance".

Tensions between the two countries have been rising for some time over the cross-border leaflets, usually sent via balloons.

South Korea's defence ministry on Tuesday responded to the renewed threats by saying it was working with the US to closely monitor military moves in the North.

What did the North say?

North and South Korea are separated by the so-called demilitarised 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 "into the zones that had been demilitarised".

The General Staff said it was on "high alert" and ready to "rapidly and thoroughly" implement any decisions by the government.

The statement comes after the sister of North Korean leader Kim Jong-un threatened military action against the South on Saturday.

"I feel it is high time to surely break with the South Korean authorities," said Kim Yo-jong, who holds a senior role in the North's political hierarchy.

She promised to take "action", said she had instructed the military, and ended her statement with: "Rubbish must be thrown into the dustbin."