The signs are unmistakable and the world is watching.

The diplomatic altercation between America and North Korea is escalating by the day.

As part of preparation to defend itself in case North Korea decides to attack, America alongside one of its strongest allies in the Korean Peninsula, the South Korea have started a joint military exercise.

On Monday, the two countries kicked off their largest ever joint air exercise, an operation North Korea has labelled an "all-out provocation".

This comes days after Pyongyang fired its most powerful intercontinental ballistic missile.

The five-day Vigilant Ace drill -- involving 230 aircraft, including F-22 Raptor stealth jet fighters, and tens of thousands of troops -- began on Monday, South Korea’s air force said.

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North Korea over the weekend blasted the drill, accusing US President Donald's Trump's administration of "begging for nuclear war".

The annual exercise comes five days after the nuclear-armed North test-fired a new ICBM, which it says brings the whole of the United States within range.

As tensions surged, US Senator Lindsey Graham, an influential Republican and foreign policy hawk, warned that the US was moving closer to "preemptive war" with the North.

"If there's an underground nuclear test (by the North), then you need to get ready for a very serious response by the United States," Graham told the CBS show "Face the Nation".

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