Taiwanese officials have arrested three Taiwanese military officers suspected of being Chinese spies, BBC reported.

Chang Chih-hsin, a retired navy officer, has been identified by local Taiwanese media as a former political warfare head of the meteorology and oceanography office, who served for 24 years. He may have been in charge of intelligence regarding classified locations of submarines, hidden ambush zones and coastal defense areas.

However, the Defense Ministry revealed that before Chang’s retirement, he had established contact with mainland Chinese officials. Chang had been under investigation prior to his retirement in May, after which he reportedly made a visit to China.

The Taiwanese Defense Ministry confirmed the arrests and has expressed grave concern.

“The Department of Defense has to do damage control. But [we] will not comment on the relevant facts. It is understood that in Taiwan, spies are taken seriously and will face the death sentence,” a spokesperson for the Defense Ministry said.

Other media reports have suggested more than three arrests have been made.

According to the Global Times, eight military officials have been arrested on espionage charges. Another one besides Chang  has been identified as Xin Zhang. Photos of him in various parts of Mainland China, including Xiamen and Fuzhou, this past August has Taiwanese media suspecting that these travels were at the root of his arrest. However, the Taiwanese Defense Ministry declined to confirm whether this was a factor.

The sensitive information that the Taiwanese officials allegedly provided to China is said to be very valuable and could be seriously detrimental to Taiwan, especially in the event of a cross-strait war.

“The locations of combat submarines, enemy zone ambush areas, as well as ocean climate and sea levels are all crucial in Taiwanese defense strategies,” the Global Times reported.

Though less turbulent than in previous years, China and Taiwan’s relationship continues to be strained as cases like these are revealed.

Taiwan has maintained informal independence from China since the end of the civil war in 1949, while the mainland China government considers the island a breakaway province.

Espionage cases are not uncommon between China and the independently governed Taiwan.

In recent history, other Taiwanese officials have been prosecuted for selling military secrets to China.

In 2011, General Lo Hsie-Che, then the head of the military command communication and information office, was sentenced to life in prison (he was spared the death penalty because he confessed). Beijing had recruited General Lo in 2004, while he was posted in Thailand as a military attaché.