North Korea has warned it is entering a "state of war" with South Korea.
It says if there is any further provocation it may deploy nuclear weapons.
North Korea has threatened attacks after it was sanctioned for a third nuclear test in February.
The US says it's taking the threats "seriously" as tensions in the region rise.
However, few think the North would risk full-blown conflict, and the two sides have technically been at war since 1953 as no peace treaty has been signed.
An armistice at the end of the Korean War was never turned into a full treaty.
North Korea's leader Kim Jong-un has been running the country since December 2011. He took over from his father Kim Jong-il who died.
The death of King Jong-il former leader of North Korea
Kim Jong-il, the former leader of North Korea died of a heart attack on a train back in December 2011. The leader of the secretive communist nation was 69.
North Korea shocked the world by announcing his death.
Kim Jong-il had been presented to his people as a father-figure and a demi-god. But his tight control of the country, and his creation of a nuclear arsenal, meant his death caused political shockwaves around the region too.
His successor, son Kim Jong-un who is thought to be in his late 20's, is largely unknown outside the secretive state, and countries throughout the region were watching closely for any instability in the handover of power.
Tension between the country and other nations - particularly its neighbour South Korea - have been running high for years.
In November 2010, South Korea scrambled fighter jets and returned fire after North Korea shot around 200 artillery shells at an island close to its border.
North Korea is a secretive country and has faced pressure from the US over fears it had plans to develop long-range nuclear missiles.
What do we know about North Korea?
Twenty-four million people live there and the government controls all TV and radio.
There's an eternal president called Kim Il-sung who led the country into the Korean war in 1950 hoping to take control of South Korea but he's dead.
Kim Il-sung's big idea was that the country could look after itself with no outside help but that led to food shortages and millions of deaths.
When he died in 1994 his son Kim Jong-il took power with state-run channels broadcasting propaganda about the leader.
They're the only stations people can watch in North Korea.
If North Koreans get caught trying to listen to foreign radio, they're likely to be punished in a labour camp. Most North Koreans are also forbidden to leave the country.
Kim's palace was reportedly surrounded by bunkers and anti-aircraft guns and other rumours included him having a collection of 20,000 films and a love of spicy food and basketball.
North Korea also qualified for the World Cup finals in South Africa, the country's first trip to the competition since 1966.
The country was named as part of the axis of evil by former US president George Bush.
In October 2010 the world's media was invited to North Korean capital Pyongyang to see a major military parade.
As well as showing off their power and criticising America, the man who now leads North Korea was unveiled to the world: Kim Jong-il's son Kim Jong-un.
What has Team America got to do with North Korea?
Team America: World Police is a comedy film released in 2004 and was written by the team behind South Park.
In it a team of US paramilitary 'policemen' try to save the world from a North Korean plot to blow up the planet's major cities.
Kim Jong-il is shown as a puppet who organises an elaborate peace ceremony to try to fool the world's leaders and Hollywood stars into coming to North Korea.
His plans fail and the North Korean leader is revealed as an alien.