The United States showed concern Wednesday about a Japanese breach of online security that has left sensitive American documents open to scrutiny. The warning from the U.S. came after it was discovered that the country’s online military and flight information, including secret data posted on the blog of an air traffic controller and flight information for Air Force One, had been tampered with.

“We are concerned by news reports of cyber attacks on select Japanese companies and will continue to monitor the issue,” said Karen Kelley, a spokeswoman for the American Embassy in Tokyo, to the New York Times.

“For every country, these kinds of intrusions have the potential for long-term negative impact and must be taken seriously,” Kelley added. “This is why cyber security must be a public sector priority in close collaboration with the private sector.”

One of the online sites attacked was Mitsubishi Heavy Industries, who said that 83 computers in 11 locations were attacked. Mitsubishi has built fighter jets, missiles and other military tools and vehicles, some of which are based in the United States.

The Japanese government has vowed to tighten its security measures, even though Defense Minister Yasuo Ichikawa has expressed no knowledge of online documents being compromised.