Defense Minister Chiu Kuo-cheng told reporters that Taiwan was cooperating with a number of friendly parties but said he could not disclose information about specific departments.

On Saturday, Japan’s Defense Ministry said the People’s Liberation Army (PLA) frigate Binzhou had passed through the Miyako Strait into the Western Pacific the day before. The Japan Maritime Self-Defense Force later deployed an Abukuma-class destroyer escort and two maritime patrol aircraft as the single Chinese warship traveled north via the narrow waters between Taiwan and Yonaguni, Japan’s westernmost inhabited island, which lies less than 70 miles from Taiwan’s east coast.

Satellite images dated May 1, however, revealed an additional detail not present in the announcement by the Japanese Defense Ministry—a Taiwanese warship also appeared to be in the vicinity, seemingly monitoring Binzhou as it returned to the East China Sea.

In the same frame, a Taiwanese navy Kee Lung-class destroyer can be seen sailing about 6 miles west of the Chinese and Japanese warships. Taipei’s Apple Daily described it as the first joint surveillance operation between Taiwan and Japan.

Taiwan’s defense officials, however, did not corroborate the newspaper’s view that Taipei and Tokyo had collaborated over the weekend.

Defense Minister Chiu said Taiwan dispatches ships and aircraft to monitor any People’s Liberation Army assets operating within designated areas.

“It’s not about specific cooperation with any country. That’s inappropriate [to disclose],” Chiu told reporters. “With regard to national defense, we will monitor whenever required.”

In a recent report sent to Taiwan’s lawmakers, the Defense Ministry said it was sharing intelligence with the American Institute in Taiwan (AIT)—the de facto U.S. embassy in Taipei—regarding Chinese navy movements in the Western Pacific and in the South China Sea.

Asked whether the ministry had shared Saturday’s operations report with the AIT, Chiu said he was “not at liberty” to name specific departments.

His deputy, Chang Che-ping, offered a similar response when quizzed by lawmakers at a defense committee hearing, also on Monday. He offered no comment on reports supposing cooperation between Taiwanese and Japanese forces.

Chang confirmed that Taiwan had tasked a Kee Lung-class destroyer to shadow Binzhou, whose crew was likely on a training and intelligence-gathering mission. Chang said the vessel “did not pose a serious threat” to Taiwan’s security.

The PLA warship’s appearance in the Western Pacific on Friday coincided with the Chinese military flying five planes into Taiwan’s air defense identification zone (ADIZ). According to Taipei’s Defense Ministry, a Chinese Y-8 anti-submarine warfare aircraft crossed the Bashi Channel, south of the island, before turning back.

Lu Li-shih, an instructor with Taiwan’s naval academy in the southern port city of Kaohsiung, said the warplane likely linked up with Binzhou for anti-submarine exercises.

Like the Miyako Strait, the Bashi Channel serves as one of the few international waterways the Chinese navy can use to exit the first island chain. Its additional strategic significance at the mouth of the South China Sea is often cited as the reason behind the PLA’s increased warplane activity in the southwest corner of Taiwan’s air defense zone.