North Korea is scrapping high-level talks with the South due to military drills with the US, reports from the region have said.
Pyongyang has also questioned US commitment to a summit between President Donald Trump and North Korean leader Kim Jong Un set for next month in Singapore.
The "indefinite postponement" of talks with South Korea came just hours before the meeting was due to start on Wednesday afternoon.
A report on North Korea's official Korean Central News Agency (KCNA) denounced the joint Max Thunder drills between the South Korean and US air forces, calling it a rehearsal for the invasion of the North.
It called the exercise a provocation amid warming inter-Korean ties, following the historical summit between the two leaders at the border village of Panmunjom on 27 April.
"This exercise … is a flagrant challenge to the Panmunjom Declaration and an intentional military provocation running counter to the positive political development on the Korean Peninsula," the KCNA report said.
"The United States will also have to undertake careful deliberations about the fate of the planned North Korea-US summit in light of this provocative military ruckus jointly conducted with the South Korean authorities."
The talks had been due to discuss follow-up measures to the leaders' summit last month.
KCNA added that Seoul and Washington had carried out the air drills before the "ink on the declaration had a chance to dry", in reference to the document calling to formally end the 1953 Korean War and for "complete denuclearisation" of the Korean peninsula.
"There is a limit to the amount of good will and opportunity we can give," the state news agency warned.
"We will be keenly watching the future behaviour of the US and South Korean authorities."
A US State Department spokesman said they had no information from the North about any threat to the summit between Mr Trump and Mr Kim, and said the department continues to plan for the meeting.
A US military statement said military exercises with South Korea are "routine and defensive in nature".
It added that the drills will run from 14-25 May.
The military drills started on Friday, with about 100 warplanes.
The US and South Korea regularly carry out military drills to practice defending the South.
However, in January Mr Trump and South Korean President Moon Jae In agreed to not hold the drills during the Winter Olympic Games in February after a diplomatic breakthrough with the North's leader.
But they agreed to resume the exercises, which involve tens of thousands of troops from both countries, after the Olympics.
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Pyongyang often reacts to the biannual drills by launching missile tests, and last April held a massive artillery exercise in retaliation.
After the US announced the military drills would resume after the Olympics this year, the North said the US should "completely halt" the drills, not just postpone them, if it "truly wishes for the improvement of North-South ties".