Anti-war activists chant slogans during a rally against South Korea-US joint military exercises in front of the US embassy in Seoul on Monday.

AFP/Seoul

North Korea fired two missiles into the sea and vowed "merciless" retaliation on Monday as the US and South Korea kicked off joint military drills denounced by Pyongyang as recklessly confrontational.

The annual exercises always trigger a surge in military tensions and warlike rhetoric on the divided peninsula, and analysts saw the North's missile tests as a prelude to a concerted campaign of sabre rattling.

"If there is a particularly sharp escalation, we could see the North orchestrating some kind of clash on the maritime border," said Jeung Young-Tae, an analyst at the Korea Institute for National Unification in Seoul.

The missile launches came with a stern warning from the nuclear-armed North Korean People's Army (KPA) that this year's military drills would bring the peninsula "towards the brink of war".

The South Korean defence ministry said the two Scud missiles were fired from the western port city of Nampo and fell into the sea off the east coast - a distance of nearly 500 kilometres.

UN resolutions prohibit any ballistic missile test by North Korea, and ministry spokesman Kim Min-Seok said Pyongyang appeared intent on triggering a "security crisis".

"We will respond sternly and strongly to any provocation," Kim told reporters.

The Japanese government said it had issued a strong protest to the North given the danger such missile launches posed to aviation and shipping.

'Brink of war'

Missile tests have long been a preferred North Korean method of expressing anger and displeasure with what it views as confrontational behaviour by the South and its allies.

"The situation on the Korean peninsula is again inching close to the brink of a war," a spokesman for the KPA General Staff was quoted as saying on Monday by the North's official KCNA news agency.

North Korea has threatened attacks, including nuclear strikes, on the US before, although it has never demonstrated a missile capability that would encompass the US mainland.

The largest element of the two South Korea-US drills that began on Monday is Foal Eagle, an eight-week exercise involving air, ground and naval field training, with around 200,000 Korean and 3,700 US troops.

The other is a week-long, largely computer-simulated joint drill called Key Resolve.

Seoul and Washington insist the exercises are defence-based in nature, but they are regularly condemned by Pyongyang as provocative rehearsals for invasion.

In a statement later on Monday, the North Korean Foreign Ministry labelled the start of the drills an act of "intolerable aggression" and said the North was ready to wage "any form of war" that the US chooses.

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