China says it has complained to France after one of its warships entered Chinese territorial waters while passing through the Taiwan Strait this month.

The April 7 incident marks a rare occasion of military friction between the two countries, which have held joint search and rescue exercises before.

Ren said the navy dispatched ships to identify, warn and escort the French ship and would remain “highly alert to firmly safeguard China’s sovereignty and security.”

The 160-kilometer (100-mile)-wide Taiwan Strait divides mainland China from the island Beijing claims as its territory. It is considered an international waterway heavily trafficked by ships from all nations, many of them bound for Chinese ports.

China is highly sensitive to operations by foreign warships near areas it claims, such as the South China Sea.