TOKYO, JAPAN -- Japan’s space agency has suffered a series of cyberattacks since last year, but sensitive information related to rockets and satellites was not affected and it is continuing to investigate and take preventive measures, officials said Friday.

Chief Cabinet Secretary Yoshimasa Hayashi acknowledged that the Japan Aerospace Exploration Agency, or JAXA, has had "a number of" cyberattacks since late last year.

JAXA has investigated the extent of illegal access while shutting down the affected networks and verifying that they did not contain classified information about operations of rockets and satellites and national security, he said.

Hayashi vowed to strengthen Japan’s ability to counter cyberattacks.

Japan has been accelerating a military buildup in response to China's growing military might and is hoping to be able to develop a counterstrike ability, but experts say Tokyo will continue to need to rely heavily on the United States for launching long-range cruise missiles at targets.

Officials on Friday said they were not aware of any information leak from the cyberattacks.

Defense Minister Minoru Kihara told reporters that the attacks on JAXA have not affected his ministry, but that he is keeping a close watch on the investigation by the agency, which is one of his ministry’s key contractors.

Education and science minister Masahito Moriyama also told a news conference Friday that he believed there was no actual damage from the cyberattacks.

Officials said JAXA is currently working with the government’s cybersecurity team to introduce countermeasures.