Two U.S. B-52 heavy bombers made a rare training flight with the Air Self-Defense Force over the Sea of Japan last week, in a mission that likely comes in response to a spate of North Korean short-range missile tests over the last several months.

The bombers, which flew from Andersen Air Force Base on the U.S. territory of Guam, conducted training with ASDF “off the coast of Japan” on Friday, U.S. Pacific Air Forces (PACAF) spokeswoman Capt. Victoria Hight said.

While she did not reveal if the mission had occurred over the Sea of Japan, aviation tracker Aircraft Spots tweeted an image of the planes’ flight path showing them into the waterway. It said three KC-135R aerial tankers also provided support for their flight.

The B-52 is what the U.S. and its allies call a “strategic asset.” The term usually refers to B-52 bombers, stealth warplanes, nuclear-powered submarines and aircraft carriers.

North Korea has lambasted the deployment of these assets, as well as joint training that features them, calling the moves “extremely provocative” and warning that they could upset the fragile detente on the peninsula.

Observers have said the U.S. has rarely sent B-52s near the Korean Peninsula since North Korean leader Kim Jong Un began making diplomatic entreaties to its neighbors and the U.S. at the beginning of last year.

But in a sign of its displeasure with stalled U.S. denuclearization talks, Pyongyang has in recent months tested a number of new short-range ballistic missiles, including weapons designed to evade South Korean and Japanese defenses, experts say.

The Pacific Air Forces also said the bombers had flown “in the vicinity of the South China Sea.”

Such training missions have apparently been ramped up amid protracted military and trade tensions between Washington and Beijing.

The U.S. sent B-52s over the disputed South China Sea — including two separate flights near some of China’s man-made islands there — within the space of 10 days in March. Beijing blasted the missions as “provocations.”

The Chinese Foreign Ministry has said no military ship or aircraft could scare Beijing away from its resolve to protect what it says is its territory.

China has built up a series of military outposts in the strategic waterway, which includes vital sea lanes through which about $3 trillion in global trade passes each year.

Washington and Beijing have frequently jousted over the militarization of the South China Sea, where China, Taiwan, Vietnam, Malaysia, Brunei and the Philippines all have competing claims.

The U.S. does not maintain any claims there but says the operations are conducted globally with the aim of promoting freedom of navigation.

The U.S. Pacific Air Forces said Thursday’s “bilateral operation was conducted to enhance combined readiness, increase air domain awareness and to ensure a free and open Indo-Pacific.”