Updated - Adds N.Korean threat of military action - South Korea fired tens of artillery rounds towards North Korea today after the North fired a projectile towards a South Korean loudspeaker that had been blaring anti-Pyongyang broadcasts, the defence ministry in Seoul said.

North Korea did not immediately respond to the South's shots, it said, as tensions rose on the peninsula.

South Korea said its detection equipment had spotted the trajectory of a suspected North Korean projectile launched at around 3:52 pm which did not appear to have damaged the loudspeaker or caused any injuries.

"Our military has stepped up monitoring and is closely watching North Korean military movements," South Korea's defence ministry said in a statement.

South Korea's military raised its alert status to the highest level.

There was no mention of the firing in North Korean state media, which does not typically make immediate comment on events.

The suspected North Korean projectile landed in an area about 60 km (35 miles) north of Seoul in the western part of the border zone, the defence ministry said. South Korean residents in the area were ordered to evacuate, according to the South's Yonhap news agency.

N. Korea threatens military action if South continues loudspeaker broadcasts

North Korea later warned of military action should South Korea continue anti-Pyongyang propaganda broadcasts along their shared border, telling Seoul to halt the loudspeaker broadcasts within 48 hours.

In a statement, the South's defence ministry said North Korea's armed forces sent a letter calling the South's propaganda broadcasts a "major challenge" to the North. 

South Korea's presidential office said it had convened a meeting of the national security council.