North Korea has fired a shell across the border into South Korea, prompting Seoul to respond with artillery fire, according to reports.
The North is believed to have been aiming at a loudspeaker in a border town that has been blaring anti-Pyongyang broadcasts recently, South Korean media said.
In response, South Korea fired tens of 155mm artillery rounds at the location where the shell came from, the country's defence ministry said.
The South's military is currently on its highest state of alert.
KBS News, which is state-run, quoted a South Korean military official as saying that North Korea opened fire at around 4pm local time (7pm GMT).
South Korea news agency Yonhap said the shots did not cause any apparent damage and that there had been no response from Pyongyang to the artillery fire.
South Korea's defence ministry said surveillance equipment detected the trajectories of suspected rockets and that it was investigating the incident.
"Our military has stepped up monitoring and is closely watching North Korean military movements," the ministry said in a statement.
Reports of evacuations
There are no details on any of the injuries in the border town or in the North, but media outlets in South Korea reported that towns on the South Korean side of the border were being evacuated.
The suspected projectile landed in an area of the demilitarised zone (DMZ) around 60kmd north of Seoul.
The projectile appeared to have landed in a mountainous area near a military base in the town of Yeoncheon, according to Yonhap.
North Korea had promised to retaliate against the broadcasts, which criticise alleged provocations from Pyongyang and praise South Korea's democracy.
Seoul restarted them after a pause of 11 years after claiming the North had laid land mines that maimed two of its soldiers.
One soldier lost both of his legs, while the other lost one. The North has also restarted its own loudspeakers.
The North's army has previously said the South's broadcasts were a declaration of war and that if Seoul did not stop them an "all-out military action of justice" would follow.
In response, South Korean President Park Geun-hye said Pyongyang should "wake up" from the delusion it could maintain its government with provocation and threats.
North Korea is highly sensitive to criticism of the government run by Kim Jong-Un, whose family has run the authoritarian country since its foundation in 1948.
The two nations are technically still at war because the 1950-1953 Korean War ended in an armistice, not a peace treaty.
The 2.5-mile wide DMZ separating the two countries is the world's most heavily armed border.
It is littered with landmines, barbed wire and heavy military equipment, as well as more than one million soldiers deployed on both sides.
Thursday's exchange of fire is the first between the two countries since October.
In that incident, soldiers from the North approached the border and did not retreat when warning shots were fired, according to the South's defence ministry.
The North Korean soldiers fired back in an exchange of fire that lasted for about 10 minutes, with no casualties.
