Chinese officials said Friday the country is watching the U.S. military closely, after two U.S. bombers flew over the disputed South China Sea.

Two U.S. Air Force B-1B Lancers assigned to the 9th Expeditionary Bomb Squadron flew a 10-hour mission from Andersen Air Force Base in Guam through the South China Sea Thursday, according to U.S. Pacific Command.

The B-1B is a long-range, multi-mission conventional bomber that has served the U.S. Air Force since the 1980s. Originally designed for nuclear strikes, the bomber is now used exclusively for conventional operations. The aircraft carries the “largest conventional payload of both guided and unguided weapons in the Air Force inventory, according to the Air Force.

During Thursday’s training exercise, part of U.S. efforts to maintain a “continuous bomber presence” in the region, the bombers were accompanied by U.S. Navy’s USS Sterett guided-missile destroyer.

“China always maintains vigilance and effective monitoring of the relevant country’s military activities in the South China Sea,” the Chinese Ministry of Foreign Affairs said in response to the overflight, “China’s military will resolutely safeguard national sovereignty, security and regional peace and stability.”

China is sensitive to American military activities in the South China Sea, especially after the Pentagon ordered the first freedom-of-navigation operation under the Trump administration in late May, sending the USS Dewey within 12 nautical miles of territory claimed by China in the Spratly Islands.

China claims the vast majority of the South China Sea, demarcating its territorial claims with a nine-dashed line. Vietnam, the Philippines, Malaysia, and Brunei, and Taiwan also have claims to the region.

The Permanent Court of Arbitration in The Hague discredited China’s claims to the South China Sea, but Beijing rejected the authority of the court, as well as the ruling.

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