Pyongyang test-fired the missile at around 5:30 a.m. (2030 UTC) near the coastal city of Sinpo, where satellite imagery shows a submarine base is located, an official at South Korea's Joint Chiefs of Staff told the Reuters news agency.

The launch came only two days after South Korea and the US had begun their annual military exercises in South Korea, which North Korea condemns as preparations for an invasion and traditionally counters with threats of retaliation. Seoul and Washington maintain the military drills are defensive in nature.

A statement from South Korea's military Joint Chiefs of Staff said the missile, launched in the early morning from a submarine in the East Sea (Sea of Japan), flew around 500 kilometers (310 miles) - a substantial improvement on similar tests in the past.

The Joint Chiefs of staff said Wednesday's test was clearly aimed at fueling those tensions and was a "serious challenge" to security on the Korean peninsula, as well as a grave breach of UN resolutions. "We will deal strongly and sternly with any provocation by the North," the statement said.

North Korea became further isolated after a January nuclear test - its fourth - and the launch of a long-range rocket in February that brought a tightening of UN sanctions.

One missile launched this month landed in or near Japanese-controlled waters.

Tensions on the Korean peninsula were aggravated by the recent defection of North Korea's deputy ambassador in London to South Korea, which has been seen as an embarrassing setback to the regime of North Korean leader Kim Jong Un.

jbh/sms (Reuters, AFP)