SEOUL, South Korea (AP) — North Korea appeared to have conducted artillery firing drills on Sunday, South Korea said, days after the United States deployed sophisticated fighter jets to South Korea for joint training.

South Korea’s Joint Chiefs of Staff said in a statement that it detected multiple trajectories presumed to be North Korean artillery on Sunday afternoon. It said South Korea’s military maintained a firm readiness in close coordination with the United States.

South Korea’s presidential office said the suspected launches occurred off the North’s west coast. It said presidential national security director Kim Sung-han reviewed South Korean military readiness and that his office closely monitors possible additional launches by North Korea.

This year, North Korea has carried out an unusually large number of weapons tests, including nuclear-capable missiles that place both the U.S. mainland and its allies including South Korea and Japan with striking distance. Some experts say North Korea attempts to perfect its weapons technology and boost its bargaining chip in future negotiations with the United States to win sanctions relief or security guarantees.

Last week, six U.S. F-35 aircraft from Eielson Air Force Base in Alaska arrived in South Korea for their first temporary deployment in South Korea in about 4 ½ years for joint training with South Korean fighter jets.

South Korea’s Defense Ministry said the jets’ deployment is aimed at demonstrating the allies’ combined defense posture and strong deterrence against potential external aggression while improving the interoperability of the two air forces. A U.S. military statement said the U.S. aircraft planned to operate over South Korea and surrounding waters during the scheduled 10-day training mission.

North Korea typically views joint exercises between U.S. and South Korea as an invasion rehearsal and responds with its own weapons tests. U.S. and South Korean defense officials have repeatedly said they have no intentions of attacking North Korea.

North Korea has said it was forced to develop nuclear weapons to cope with U.S. military threats. Despite its torrid run of missile tests earlier this year, North Korea hasn’t conducted its expected first nuclear bomb test in five years, and Seoul officials said that’s likely because of an ongoing COVID-19 outbreak and opposition by China, its last major ally and aid benefactor.