The latest joint exercises come at a time of heightened tensions surrounding Pyongyang's nuclear and missile programs, with fears of a possible military clash following recent exchanges of insults and fierce rhetoric between President Trump and North Korean leader Kim Jong Un.
South Korean and U.S. forces regularly conduct joint drills, and North Korea condemns them as invasion rehearsals.
South Korea's navy says the exercises that began Monday in the waters off the Korean Peninsula involve fighter jets, helicopters and 40 naval ships, including the aircraft carrier USS Ronald Reagan. Spokesman Jang Wook told reporters the drills are aimed at practicing how to respond to a potential naval provocation by North Korea and improving the allies' combined operational capability.
The drills were to include live-fire exercises by naval ships and aircraft and anti-submarine training, but South Korea's military didn't release any photos or video.
North Korea didn't immediately respond to the start of the drills.
Pyongyang last week accused the U.S. of provoking it by mobilizing the aircraft carrier and other war assets near the peninsula.
The North said it could take military counteraction such as a salvo of missile launches into waters near Guam.
Lobbing missiles close to Guam would be deeply provocative for the United States, and a miscalculation on either side could lead to a military confrontation.
Under Kim's leadership, Pyongyang has been accelerating its efforts to bolster its weapons arsenals and acquire the capability to fire nuclear missiles at any target in the U.S. mainland. The North conducted its sixth and most powerful nuclear test in September and test-launched two intercontinental ballistic missiles in July.
On Sunday, U.S. Secretary of State Rex Tillerson said diplomatic efforts to resolve the North Korean crisis "will continue until the first bomb drops."
That statement comes despite Mr. Trump's tweets a couple of weeks ago that his chief envoy was "wasting his time" trying to negotiate with "Little Rocket Man," a mocking nickname Mr. Trump has given the nuclear-armed nation's leader Kim Jong Un.
"I think he does want to be clear with Kim Jong Un and that regime in North Korea that he has military preparations ready to go and he has those military options on the table. And we have spent substantial time actually perfecting those," Tillerson told CNN's "State of the Union. But be clear: The president has also made clear to me that he wants this solved diplomatically. He's not seeking to go to war," Tillerson said.
Recent mixed messaging from the top of the U.S. government has raised concerns about the potential for miscalculation amid the increasingly bellicose exchange of words by Trump and the North Korean leader.
But Tillerson said on the CBS News broadcast "Face the Nation" Sunday there is "absolute alignment" between the State Department and White House on policies North Korea and Iran.