The Vietnam Post, the country’s central postal service, has strongly objected to China’s recent issuance of stamps that violate Vietnam’s sovereignty over Truong Sa (Spratly) archipelago in the East Vietnam Sea.

On October 28, the Chinese postal service issued a collection of postage stamps, which feature five lighthouses it has built on Chau Vien, Gac Ma, Si Bi, Chu Thap and Vanh Khan– reefs on Vietnam’s Truong Sa (Spratly) archipelago that are being occupied illegally by China.

The Vietnam Post said in a statement on Tuesday that the stamp collection is a serious violation of Vietnam’s sovereignty over Truong Sa archipelago, and runs counter to Article 8 of the Universal Postal Union Convention.

The Vietnamese postal service therefore demanded that China immediately annul all stamps, envelopes and publications featuring islands on Truong Sa archipelago.

Also on Tuesday, the Ministry of Information and Communications, which manages the Vietnam Post, released a separate statement to protest against China’s stamp issuance.

The ministry said in the document that Vietnam has full historical and legal evidence affirming its sovereignty over Hoang Sa (Paracel) and Truong Sa archipelagoes, urging China Post to respect the historical truth.

The Vietnamese ministry also asked China Post not to repeat the action in order to foster bilateral ties between the two post agencies, according to the Vietnam News Agency.

This is not the first time China Post issued stamp collection that violated Vietnam’s sovereignty.

In May 2013, on the anniversary of China’s Tourism Day, the Chinese organization issued a set of six stamps named “Beautiful China” featuring six tourist attractions. However, only five of these sites are in China, whereas the other belongs to Vietnam’s territorial sovereignty.

The stamp issuance also sparked protest from the Vietnamese side.