North Korea has fired dozens of artillery shells at a South Korean island, setting buildings on fire and prompting a return of fire by the South.
Following are some details about the armed forces deployed on the Korean peninsula.
North Korea has 1.19 million troops in active service, and more than 7.7 million reservists. It is one of the world's most militarised countries with a population of 23.4 million.
South Korea has 655,000 troops on active duty, and about 3 million reserve forces. They are reinforced by 28,000 US troops stationed in the South.
North Korea has some 4,000 tanks and more than 2,100 armoured combat vehicles. Much of the equipment is believed to be Soviet-era procurement and in need of upgrading. The T-54, the North's main combat tank, began to be phased out by most other countries in the 1970s.
South Korea has 2,300 tanks and 2,400 armoured vehicles.
Soviet-made MiGs make up the bulk of the North's air force. However, the fleet is largely obsolete and not fit for modern combat. The South has about 490 combat aircraft.
North Korea is believed to be steadily building its submarine fleet, with its 70 vessels outnumbering the South's dozen or so. Its 420 warships also outnumber the South's roughly 120 vessels, but the South has been adding powerful destroyers to its fleet.
North Korea has limited fuel supplies, and relies heavily on China for its crude oil and gasoline.
North Korea has more than 800 ballistic missiles and more than 1,000 missiles of various ranges. It has sold missiles and technology overseas, with Iran a top buyer. South Korea is limited in pursuing missile development under a treaty with the United States but has recently deployed new long-range cruise missiles with a range of 1,500 km which can hit all of North Korea and also targets in China and Russia.
North Korea is believed to have produced about 50 kg (110 lb) of plutonium, which experts say would be enough for six to eight nuclear weapons. It has twice conducted nuclear tests but has yet to show that it has a working nuclear bomb.
South Korea, an advanced nuclear power state, does not have a nuclear arms programme, although Washington has promised protection under its "nuclear umbrella".
US FORCES IN KOREA
Its main frontline 2nd Infantry Division is armed with 140 M1A1 Abram battle tanks, 170 M2 Bradley Fighting vehicles, rocket launchers, tactical missiles, Patriot missile defence systems.
The US Air Force operates F-16 fighters, ground attack planes and three U-2 spy planes.