North Korea fired missiles into the sea Monday and vowed "merciless strikes" against the US and South Korea, as the allies kicked off eight weeks of joint military drills denounced by Pyongyang.
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.
The launches came with a stern warning from the nuclear-armed North Korean People's Army (KPA) that this year's drills would bring the peninsula "towards the brink of war."
The South Korean military said the two North Korean missiles, with a range of around 500 kilometres (310 miles), were fired from the western port city of Nampo and traversed the country before falling into the sea off the east coast.
Describing the tests as a "provocation" in response to the start of the military exercises, the Joint Chiefs of Staff said in a statement that it remained "vigilant against any additional launches."
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 Monday by the North's official KCNA news agency.
"The only means to cope with the aggression and war by the US imperialists and their followers is neither dialogue nor peace. They should be dealt with only by merciless strikes."
North Korea has threatened attacks, including nuclear strikes, on the US before, although it has never demonstrated a missile capability that would reach the US mainland.
- Retaliation threat -
Although its nuclear program remains shrouded in uncertainty, Pyongyang is currently believed to have a stockpile of some 10 to 16 nuclear weapons fashioned from either plutonium or weapons-grade uranium.
A new research report by US experts published this week estimated that North Korea could be on track to have an arsenal of 100 nuclear weapons by 2020.
The largest element of the two South Korea-US drills that began 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.
"Our revolutionary armed forces will never remain a passive onlooker to this grave situation," the KPA spokesman said, adding that North Korea would respond in kind to any conventional, nuclear or cyber act of war against it.
"In case even a single shell drops on any place over which the sovereignty of the DPRK (North Korea) is exercised, it will promptly take counteractions."
North Korea has carried out three nuclear tests -- in 2006, 2009 and 2013.
In January, the North offered a moratorium on further tests if this year's joint drills were cancelled -- a proposal rejected by Washington as an "implicit threat" to carry out a fourth atomic detonation.