Even before the sub surfaced in Busan, North Korean officials were warning the U.S. against “foolish” actions, with Workers Party official Kim Yo Jong, the sister of North Korean leader Kim Jong Un, saying through state media on Tuesday that North Korea had launched a “military offensive” in response to U.S. aggression.

On Wednesday, North Korea launched two short-range ballistic missiles into the East Sea. In a statement Thursday, North Korea’s defense minister Kang Sun-nam said that the presence of the sub “may fall under the conditions of the use of nuclear weapons.”

South Korean officials have denounced North Korea’s launches as a “major provocation.”

American officials said earlier this week the deployment of the submarine was part of its “extended deterrence” policy.

“This port visit to Busan reflects the United States’ ironclad commitment to the Republic of Korea for our extended deterrence guarantee, and complements the many exercises, training, operations, and the other military cooperation activities conducted by Strategic Forces to ensure they are available and ready to operate around the globe at any time,” U.S. Forces Korea said in a news release Tuesday.

The spat over the submarine comes as the United States, the U.S.-led United Nations Command and the U.S.’s Swedish interlocutors seek information from Pyongyang on Travis King, the American soldier who crossed into North Korea while on a civilian tour of the Demilitarized Zone on Tuesday. In a virtual gaggle Thursday, Kirby told reporters the United States had not received any information on King, either through Sweden or other channels.

U.S. officials maintain they are still working to get information from North Korea about King’s well-being, despite the silence from Pyongyang. Aboard Air Force one Thursday, White House deputy press secretary Olivia Dalton told reporters the U.S. government has been reaching out through “multiple channels” to speak with North Korean officials.