South Korea kicked off its annual joint military exercises with the United States, despite vocal opposition from North Korea.
Monday's start of this year's drills overlaps with the first reunion of families divided by the Korean War in three years, an event that has raised hopes of greater North-South cooperation, AFP news agency reports.
Pyongyang had initially insisted that the joint exercises be postponed until after the reunion finishes on Tuesday, but Seoul refused and in a rare concession, the North allowed the family gathering to go ahead as scheduled.
The annual "Key Resolve" and "Foal Eagle" drills, routinely condemned by North Korea as rehearsals for invasion, will last until April 18 and involve a combined total of 12,700 US troops and many more from South Korea.
"Key Resolve" lasts just over a week and is a largely computer-simulated exercise, while the eight-week "Foal Eagle" drill involves air, ground and naval field training.
Seoul and Washington insist they are both defensive in nature, playing out various scenarios to combat a North Korean invasion.
Last year's drills fuelled an unusually sharp and protracted surge in military tensions, with Pyongyang threatening a pre-emptive nuclear strike, and nuclear-capable US stealth bombers making dummy runs over the Korean peninsula.
US defence officials have indicated that this year's drills will be slightly toned down, with no aircraft carrier or strategic bombers.
However, South Korean Defence Ministry spokesman Kim Min-Seok insisted Monday there would be "no readjustment" in the scale of the manoeuvres.