Chinese warships locked their fire-control radar on a Maritime Self-Defense Force destroyer and a helicopter on two separate incidents last month in the East China Sea, prompting the government Tuesday to denounce the “very dangerous act” and lodge a formal protest with Beijing.

Defense Minister Itsunori Onodera held a hastily arranged news conference Tuesday evening to reveal the incidents, which took place on Jan. 19 and 30, saying the ministry had just finished analyzing the radar from the Chinese frigates Jiangkai I and the Jiangwei II.

After what it described as a careful study of the data, the ministry concluded the signals came from the Chinese ships’ fire-control systems and were aimed at the MSDF ship and helicopter.

Onodera denounced the “very abnormal” acts and said they “could develop into a very dangerous situation.”

“Usually any country that owns ships like these won’t (direct the radar) of their fire-control system (on ships of another country) unless in extreme circumstances,” Onodera said.

The fire-control system is used to aim a warship’s weapons, including missiles.

The fire-control “radar of a ship is activated before the use of weapons. Directing such radar signals at another party is a dangerous act that could lead to unpredictable situations,” a senior Defense Ministry official said.

Prime Minister Shinzo Abe on Wednesday condemned China over the recent locking of a weapons-targeting radar but told a Diet session that China and Japan should maintain a “mutually beneficial strategic relationship” and try to prevent such incidents from happening and escalating.

“It was a unilateral, provocative and dangerous act, and extremely regrettable,” Abe told an Upper House session, adding such target-lock action could have triggered a crisis.

“I urge strong restraint by China so the situation will not unnecessarily escalate,” Abe said, stressing the need for the two countries to go back to the principle of a “strategic relationship of mutual benefit.”

Onodera on Tuesday did not reveal where the two incidents took place because that is classified information pertaining to MSDF operations.

But Kyodo News quoted a source in the ruling coalition as saying both incidents took place near the Senkaku Islands, known as Diaoyu in China, which are controlled by Japan but claimed by China and Taiwan.

According to Defense Ministry officials, the 3,953-ton Jiangkai I emitted radar signals at around 5 p.m. Jan. 19 that set off the threat-alarm system onboard a SH-60K helicopter from the MSDF destroyer Onami.

The chopper was flying several kilometers from the ship over the high seas, the ministry said.

At around 10 a.m. Jan. 30, the MSDF destroyer Yudachi detected the fire-control system radar from the Jiangwei II, which was about 3 km away, over the course of several minutes, the ministry said.

Both incidents immediately “raised the sense of tension” among the MSDF crews, Onodera said.

Neither of the Chinese ships tried to communicate with the MSDF by VHF, as is customary among navies worldwide, according to the Defense Ministry.

In Beijing on Wednesday, the Chinese Foreign Ministry said it is “not aware” of details about the incidents, indicating the action was at the Chinese navy’s initiative.

Meanwhile, the U.S. Department of State spokeswoman Victoria Nuland on Tuesday expressed concerns about the actions by the Chinese warships.

“I will say that with regard to the reports of this particular lock-on incident, actions such as this escalate tensions and increase the risk of an incident or a miscalculation,” she told a daily briefing in Washington.

“They could undermine peace, stability and economic growth in this vital region. So we are concerned about it,” she said.

Katsunobu Kato, deputy chief Cabinet secretary and a close aide to Abe, told reporters Wednesday that Beijing and Tokyo should “control situations” so individual incidents will not affect what he and Abe describe as “one of the most important bilateral relationships” for Japan.

“We’d like to handle the relationship with China from a broad perspective,” he said, adding Tokyo and Beijing should establish a hotline to prevent maritime confrontations from escalating.

Relations with China have been severely strained since the central government effectively nationalized the Senkakus in September.