Four U.S. stealth fighter jets flew over South Korea on Wednesday amid escalated tensions with neighboring North Korea. The F-22s were seen landing at Osan Air Base near Seoul and were escorted by other U.S. and South Korean fighters, the AP reports. The U.S. has previously sent the warplanes, which are capable of escaping radar undetected, to South Korea during times of escalation with the North. The move comes a day after South Korean President Park Geun Hye warned of the North’s “regime collapse” if it didn’t end its nuclear program. The U.S. military would not say how long the F-22s would remain in South Korea.