Operation Max Thunder involves 11 days of joint manoeuvres featuring aircraft carriers, fighter jets and advanced military drills.
Around 1,000 US service personnel are taking part in the annual training exercise alongside 500 South Korean troops.
The Trump administration has stressed Max Thunder is a routine drill planned months ago.
A US pilot gestures from an F-16 jet fighter before taking off during Operation Max Thunder
The U.S. and South Korea Launch Operation Max Thunder
Thu, April 20, 2017
The US and South Korea kicked off a big exercise just as North Korea threatens 'thermonuclear war'. Exercise 'Max Thunder' is to 'enhance interoperability' between the US and South Korean forces
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A South Korean F-15K jet fighter takes off during the 'Max Thunder' South Korea-US military joint air exercise at a US air base in the southwestern port city of Gunsan
North Korea routinely labels such military exercises as preparations for invasion and war.
And its timing coincides with increased threats from the hermit state's dictator Kim Jong-un who has vowed to “reduce the USA to ashes” in a “pre-emptive thermonuclear missile strike” idk the provocation continues.
Kim’s bloodcurdling warnings are testing the patience of Washington’s hardliners who want to stamp out the loony leader’s nuclear and missile programmes before he gets chance to use them.
President Donald Trump has tweeted: “Our military is building and is rapidly becoming stronger than ever before. Frankly, we have no choice!”
F16 fighters of the USA Air Force are seen on the runway during Operation Max Thunder
I am sure we can deter war and remove our enemy's intention to provoke us
Colonel Lee Bum-chul
Lt. Gen. Thomas W. Bergeson, 7th Air Force commander, said: “Exercise Max Thunder serves as an invaluable opportunity for US and South Korean forces to train together shoulder-to-shoulder and sharpen tactical skills vital to the defence and security of the Korean Peninsula.
“This exercise will rigorously test our aerial combat capability and highlights the ironclad commitment between the US and South Korea and the multifaceted capabilities we possess in this theatre.”
South Korean pilot Colonel Lee Bum-chul said: "We are conducting a practical and more intensive exercise than ever.
"Through this exercise, I am sure we can deter war and remove our enemy's intention to provoke us."
US soldiers stand guard during the Max Thunder South Korea-US military joint air exercise
The joint training mission got under way after North Korea’s latest missile test ended in embarrassing failure when the rocket veered wildly off course and exploded just seconds after it was launched in the early hours of Sunday morning.
The reclusive state’s media has made no comment on the launch, which analysts say is normal for the Pyongyang regime when things go awry.
Choe Ryong Hae, a close aide to Kim, said: “If the United States wages reckless provocation against us, our revolutionary power will instantly counter with annihilating strike, and we will respond to full-out war with full-out war and to nuclear war with our style of nuclear strike warfare.”
South Korean F-15K jet fighters taxi for take-off
Analysts do not believe Mr Trump is seeking to become embroiled in hostilities with Kim’s crackpot regime but to “harden” China's position towards the reclusive state by unnerving Beijing into reining in its wayward neighbour.
South Korea's acting president Hwang Kyo-ahn has called for the military and security ministries to maintain vigilance across the troubled region.