Criticizing the allies' joint drills and adoption of high-tech weapons, North Korea has fired a series of missiles and rockets since leader Kim Jong Un and US President Donald Trump agreed at a June 30 meeting to revive stalled denuclearization talks.
Trump has played down the tests by saying they did not break any agreement he had with Kim, but the talks have yet to resume, and analysts believe the tests are designed both to improve North Korean military capabilities and to pressure Washington to offer more concessions.
The US Department of Defense will "continue to monitor the situation and are consulting closely with our South Korean and Japanese allies," a department spokesman said.
North Korea remains unchanged in its commitment to resolve the issues through dialog, but "will be compelled to seek a new road as we have already indicated," if South Korea and the United States continue with hostile military moves, a North Korean foreign ministry spokesman said in a statement released through state news agency KCNA.
The arrival of new, US-made F-35A stealth fighters in South Korea, the visit of an American nuclear-powered submarine to a South Korean port, and US tests of ballistic missiles, are among the steps that have forced North Korea to continue its own weapons development, the spokesman said.
"The US and South Korean authorities remain outwardly talkative about dialog," the spokesman said. "But when they sit back, they sharpen a sword to do us harm."
A South Korean Joint Chiefs of Staff spokesman told a regular press briefing on Monday that the allies were "preparing for a joint exercise in the latter half of the year," but would not confirm the name of the exercise or whether it has already started.
South Korean media reported that US-South Korea joint military exercises had de facto begun on Monday, to verify the South Korean military's basic operational capability for the transfer of wartime operational control.
Pyongyang has "continued to enhance its nuclear and missile programs" and used cyberattacks to take in $2 billion to fund the development, a United Nations report said on Monday.