SEOUL, South Korea  Escalating its anti-foreign vitriol, North Korea threatened on Wednesday to shoot down American military planes it said were spying on the site of its impending rocket launching.

In mid-March, North Korea’s regime announced that it would launch a satellite into orbit between Saturday and next Wednesday. Since then, it has warned that any attempt to interrupt the launching will be considered an act of war.

“If the brigandish U.S. imperialists dare to infiltrate spy planes into our airspace to interfere with our peaceful satellite launch preparations, our revolutionary armed forces will mercilessly shoot them down,” the North’s state-run Korea Central Radio said.

The broadcast accused the United States of deploying RC-135 surveillance aircraft to spy on the Musudan-ri launch site near the North’s northeast coast. North Korea regularly accuses American surveillance aircraft of intruding into its airspace, a claim the United States military dismisses as propaganda. Its threats to shoot them down reflects the overall tension between the United States and North Korea over the planned rocket launching.