South Korean and American forces have begun joint naval exercises off the east coast of the Korean peninsula.
The drills, which follow a threat by North Korea to test a third nuclear device, are scheduled to last three days and have been described as pre-planned.
A nuclear-powered US submarine has joined warships and other vessels taking part.
North Korea has expressed anger at the exercises.
In December the United Nations expanded sanctions against North Korea after it launched a long-range rocket.