South Korea's defense ministry has said that Seoul and Washington have begun a week-long joint military exercise in waters around the Korean Peninsula, against a backdrop of high tensions due to the DPRK's nuclear and missile programs.
Around 40 vessels from the two countries, including the nuclear-powered aircraft carrier USS Ronald Reagan, are taking part.
The DPRK has called the drills a "rehearsal for war".
Tensions on the Korean Peninsula have risen sharply in recent weeks, following a series of weapons tests by Pyongyang.
On Sunday, US Secretary of State Rex Tillerson said President Donald Trump had instructed him to continue diplomatic efforts to ease tensions.