China turned down a port call request by a U.S. aircraft carrier in Hong Kong Thursday evening, the South China Morning Post reported Friday. The Chinese Foreign Ministry reportedly informed the Americans that the USS John C. Stennis would not be allowed to dock, an unnamed U.S. Consulate official told the Hong Kong newspaper. The refusal comes as tensions are rising in the South China Sea.

“[The ministry] needs to approve every ship coming into Hong Kong. [They] said ‘no’ to the carrier,” the official said. The reason for the denial was not made clear.

China is engaged in territorial disputes in the South China Sea with the Philippines, Vietnam and other countries in the region. In a move analysts said likely angered Beijing, U.S. Secretary of Defense Ash Carter visited the USS Stennis earlier this month as it sailed near the Philippine island of Luzon. 

“I get to give a message to the troops of how much we appreciate what they're doing out here to keep peace and stability in this region,” Carter said during a news conference with reporters during the visit. “The United States intends to continue to play a role out here that it has for seven decades — keeping peace and stability, which has allowed the Asian miracle of prosperity and political and economic development, each according to their own wishes, to happen.”

The South China Sea is an economically vital trade route, with a number of states laying claim to parts of it. China has asserted the farthest-reaching claims, building islands and conducting naval patrols to assert its control. The region has been a flashpoint for tensions for decades. While China has sought bilateral, closed-door negotiations to ease tensions, other countries have generally pushed for international mediation. 

The Foreign Ministry told the South China Morning Post Friday that U.S. port calls for warships and aircrafts are decided on a case-by-case basis.