North and South Korea have fired a number of missiles into waters near each other's coasts in a marked escalation of hostilities.

The North launched its most missiles in a single day - at least 23 - including one that landed less than 60km (37 miles) off the South's city of Sokcho.

Seoul responded with warplanes firing three air-to-ground missiles over the disputed maritime demarcation line.

Later Pyongyang fired six more missiles and a barrage of 100 artillery shells.

The North says the launches are in response to large-scale military exercises current being held by South Korea and the United States, which it calls "aggressive and provocative".

On Tuesday Pyongyang warned they would pay "the most horrible price in history" if they continued their joint military drills, seen as a veiled threat to use nuclear weapons.

Wednesday's exchanges began with missile launches by Pyongyang into waters close to South Korea, triggering air raid sirens on Ulleung, an island controlled by Seoul. Residents there were told to evacuate to underground shelters.

One ballistic missile crossed the Northern Limit Line (NLL), a disputed maritime border between the Koreas.

It landed outside South Korea's territorial waters but it's the closest a North Korean missile has come.

Seoul called it an "unacceptable" breach of its territory.

Officials there said the air-to-ground missiles fired by the South landed a similar distance past the NLL off the North's coast.

The tit-for-tat missile launches come during a period of national mourning in South Korea, following the crowd crush in Seoul at the weekend which killed more than 150 people.