The U.S. and South Korean militaries began a massive joint drill on Monday that'll run for five days and feature hundreds of warplanes taking part in attack preparation scenarios 24 hours a day, per Reuters.

The big picture: Operation Vigilant Storm builds on the countries' largest joint military exercises since 2017 in August, launched in the face of threats from North Korea's military. The air forces will this week conduct about 1,600 sorties — "the largest number ever for this annual event," according to a U.S. Air Force statement.

• Pyongyang has slammed the joint exercises as a "rehearsal for invasion and proof of hostile policies by Washington and Seoul," but the allies say the drills are required due to the North Korean military launching a record number of missiles this year, Reuters notes.
• North Korean leader Kim Jong-un has also strengthened the country's nuclear policy and said earlier this month that his military's spate of recent missile launches were "tactical nuclear" drills that represented a "warning" to the U.S. and South Korea.

State of play: The South Korean and U.S. Air Forces "will work together with the joint services to perform major air missions such as close air support, defensive counter air, and emergency air operations 24 hours a day during the training period" through Friday, according to the U.S. Air Force statement.

• "Support forces on the ground will also train their base defense procedures and survivability in case of attack."

Editor's note: This article has been updated with more details on the latest exercises and further context.