South Korean defense officials said the joint exercises will last 11 days while some field training will continue through late April.
The drills are the allies' first major combined military exercises since North Korea shelled a front-line South Korean island in November, killing four people.
That barrage came eight months after 46 sailors were killed when a South Korean warship was sunk, which a Seoul-led international investigation blamed on a North Korean torpedo attack; Pyongyang denies involvement. Animosity over the bloodshed drove ties between the Koreas to one of their lowest levels in decades.
Meanwhile, about 20 South Korean peace activists protested the joint military drills, which are aimed at defending South Korea and responding to any attack. About 12,800 U.S. troops and some 200,000 South Korean soldiers and reservists were said to participate in the drills.
Hours after the exercises started, North Korea warned of a nuclear war on the peninsula.
"It's an anti-national scheme aimed at prolonging the stage of confrontation and tension to realize a plot to start a northward invasion," the North's main Rodong Sinmun newspaper said in a commentary carried by the official Korean Central News Agency. "The danger of a nuclear war on the Korean peninsula is deepening."
On Sunday, the North said that if provoked, its military would turn Seoul into a "sea of flames" and start a full-scale war with "merciless" counterattacks.
South Korean and U.S. officials have repeatedly said the drills are purely defensive.
The Associated Press contributed to this report.