SEOUL, South Korea (AP) — North Korea on Thursday threatened the “toughest reaction” to the United States’ expanding joint military exercises with South Korea to counter the North’s growing nuclear weapons ambitions, claiming that the allies were pushing tensions to an “extreme red line.”

The statement by Pyongyang’s Foreign Ministry came in response to comments by US. Defense Secretary Lloyd Austin, who said in Seoul on Tuesday that the United States would increase its deployment of advanced military assets to the Korean Peninsula, including fighter jets and aircraft carriers, as it strengthens joint training and operational planning with South Korea.

South Korea’s Defense Ministry said the United States flew B-1B bombers and F-22 and F-35 fighter jets in an exercise with South Korean fighters on Wednesday above South Korea’s western waters, which it said was aimed at demonstrating the United States’ commitment to defend its ally with the full range of its military capabilities. The United States and South Korea are also planning to hold a simulation exercise this month aimed at sharpening their response if North Korea uses nuclear weapons.

In a statement attributed to an unidentified spokesperson of its Foreign Ministry, North Korea said the expansion of the allies’ drills is threatening to turn the Korean Peninsula into a “huge war arsenal and a more critical war zone.” The statement said the North is prepared to counter any short-term or long-term military challenge by the allies with the “most overwhelming nuclear force.”

“The military and political situation on the Korean Peninsula and in the region has reached an extreme red line due to the reckless military confrontational maneuvers and hostile acts of the US and its vassal forces,” the spokesperson said.

North Korea for decades has described the United States’ combined military exercises with South Korea as rehearsals for a potential invasion, although the allies have described those drills as defensive.

North Korea last year ramped up its own weapons demonstrations as the allies resumed their large-scale training that had been downsized for years.

North Korea’s actions included a slew of missile and artillery launches that it described as simulated nuclear attacks on South Korean and US targets.

“DPRK will take the toughest reaction to any military attempt of the US on the principle of ‘nuke for nuke and an all-out confrontation for an all-out confrontation!’” the North Korean spokesperson said, invoking the country’s formal name, the Democratic People’s Republic of Korea.

“If the US continues to introduce strategic assets into the Korean Peninsula and its surrounding area, the DPRK will make clearer its deterring activities without fail according to their nature,” the spokesperson said.

Austin’s visit came as South Korea seeks stronger assurances that the United States will swiftly and decisively use its nuclear capabilities to protect its ally in face of a North Korean nuclear attack.