South Korea and the United States began week-long joint navy drills in the waters around the Korean peninsula on Monday, amid high tensions over North Korea’s nuclear and missile program.

About 40 ships from both countries, including the nuclear-powered USS Ronald Reagan aircraft carrier, are taking part in the exercises on the east and west coasts of the peninsula from Oct. 16 to Oct. 20, a spokesman for the South’s defence ministry said on Monday.

North Korea has called joint military exercises by the United States and South Korea a “rehearsal for war”.

On Sunday, US Secretary of State Rex Tillerson said that President Donald Trump had instructed him to continue diplomatic efforts to calm rising tensions with North Korea, saying “those diplomatic efforts will continue until the first bomb drops.”

Tensions on the Korean peninsula have risen sharply in recent weeks following a series of weapons tests by Pyongyang, including its sixth and most powerful nuclear test on September 3 and two missile launches over Japan.

North Korea is preparing to test a long-range missile which it believes can reach the west coast of the United States, said a Russian lawmaker who returned from a visit to Pyongyang earlier this month.

South Korean intelligence officials and analysts have said that North Korea might time its next provocation to coincide with China’s Communist Party Congress which begins on Wednesday.

Speaking at a military conference in Seoul on Monday, General Terrence J. O’Shaughnessy, commander of the US Pacific Air Force, said North Korea’s nuclear weapons and missile development was “truly a threat to us all” and the United States remains poised to defend allies.

“Although countries like North Korea threaten regional peace and security, our allied air power must be ready to respond with rapid, lethal and overwhelming force to respond to preserve our interests,” O’Shaughnessy said.