North Korea has backed communist insurgencies around the world, but never to the extent in which the U.S. was involved in global conflicts.
The four-day drills are set to conclude Tuesday.
South Korea said its military is maintaining combat readiness.
NATO "has intensified operative and combat training activities, including nuclear weapons use procedures," Russian Defense Minister Sergey Shoigu said.
The NATO chief said Kim Jong Un's potentially nuclear-capable missiles threaten Europe, but North Korea said it's the U.S. that's at risk.
Trump thinks Putin can help pressure North Korea.
Defectors claim that 80 percent of trees planted near the test site die, that underground wells have been evaporated and that babies in the region are born with birth defects at alarming rates.
Japan is the only country ever attacked with a nuclear weapon, and more than 72 years later it has the ability to build a bomb of its own.
A war with North Korea could lead to millions of deaths.
Americans are more fearful of North Korea in the Trump era.
North Korea's state-run media said President Donald Trump "absolutely" needed medication to avoid starting a nuclear war.
President Trump will address the North Korea threat during his upcoming trip to Asia.
With the exception of George W. Bush, every president since Ronald Reagan has made at least one stop to the DMZ.
Senator Chris Murphy is worried about President Donald Trump's stance toward North Korea.
"It cannot be that North Korea is the only Korean country in the world that has nuclear weapons."
Pyongyang's September nuclear test was its most powerful yet.
"The thing that scares me the most in the long term is the state of our politics here, and...it's gotten worse," Michael Morell said.
This would be North Korea's seventh nuclear test.
President Donald Trump needs Congressional authorization to order military action against a foreign country, but he probably doesn't care.
"The country has steadfastly improved its nuclear and missiles capability," Defense Minister Itsunori Odonera said.
Trump has said military options are on the table if Pyongyang doesn't end its nuclear weapons program.
The monster-themed ballistic missile is part of a three-tier attack plan designed to inflict "unbearable cost" to North Korea's military and government.
Analysts say North Korea may have to move on to other tunnel testing sites or build new ones.