The U.S. and South Korea conducted joint naval exercises in the Sea of Japan on Monday amid growing tensions with North Korea over its nuclear weapons program.

The operation comes after Pyongyang’s fifth and most powerful atomic test earlier this month — its second of the year — and was intended to “send a strong message of unified resolve,” the U.S. and South Korea said in a joint statement.

The drills appeared to be the first coordinated operation by the allies outside of routine exercises since the Sept. 9 nuclear test.

The U.S. Arleigh Burke-class guided-missile destroyer USS Spruance was joined by Aegis destroyers, submarines, anti-submarine helicopters and U.S. and South Korean P-3 patrol aircraft in the operation to highlight the allies’ defense from surface, subsurface, and ballistic missile threats.

“We work side-by-side with our ROK partners every day; we are by their side today at sea, and we will remain by their side to defend against North Korea’s unprovoked acts of aggression,” Rear Adm. Brad Cooper, commander of U.S. Naval Forces Korea and Task Force 78, said in the statement.

ROK is the acronym for the Republic of Korea, South Korea’s official name.

Last Wednesday, the U.S. flew two supersonic bombers over South Korea — with one landing on the Korean Peninsula for the first time in 20 years. One of the bombers also flew the closest a B-1B strategic bomber has ever flown to the border between the North and South.

The flight was the second time in less than a month that the United States, which has about 28,500 troops in South Korea, flew bombers over the country.

Pyongyang has ratcheted up tensions on the peninsula in recent weeks with the atomic test and a slew of ballistic missile tests, including a volley of three fired while China was hosting a Group of 20 summit in the southeastern city of Hangzhou. The three missiles fell within Japan’s exclusive economic zone in waters west of Hokkaido.

The reclusive nation also claimed to have successfully test-fired a submarine-launched ballistic missile (SLBM) on Aug. 24, when the foreign ministers of Japan, China and South Korea met in Tokyo to discuss regional issues, including the North’s nuclear saber-rattling.

North Korean leader Kim Jong Un called that test “the greatest success” after the missile landed in Japan’s air defense identification zone, flying about 500 km — the longest flight by a weapon of that type.

The SLBM test has left the three allies concerned that further development successes could give the North a difficult-to-detect weapon that would pose serious security threats to the U.S., Japan and South Korea.

However, while the SLBM test suggests the program may be progressing faster than originally expected, experts say this does not mean it will be ready immediately.

“Rather, the pace and method of the North’s SLBM testing would suggest possible deployment in an initial operational capability by the second half of 2018 at the earliest,” John Schilling, a California-based aerospace engineer and expert on North Korea’s missile program, wrote on the influential 38 North website last month.

In the meantime, the U.S. and South Korea are expected to work together to better react to the threat.

“Our ROK-U.S. alliance will counter the North Korean nuclear development and SLBM threats with determination,” South Korean fleet commander Vice Adm. Lee Ki-sik said in Monday’s statement.

In another demonstration of force, Washington is planning to dispatch a nuclear-powered aircraft carrier to South Korea next month as part of its efforts to deter further military provocations by the North, reports have said.

South Korea’s Yonhap news agency has said that the Yokosuka, Kanagawa Prefecture-based USS Ronald Reagan will participate in a joint naval exercise with the South Korean military from Oct. 10 to 15.

It said the exercise will focus on training the allies’ naval forces in joint precision attacks on the North’s military facilities and the regime’s leadership that will be launched in the event of a war with the isolated state.