China is closely monitoring the activities of other counties' military forces in the South China Sea, Chinese Defense Ministry said in a statement Friday.
The Defense Ministry's statement followed the reports about the United States conducting aviation drills in the South China Sea.
According to the announcement made by US Pacific Command on Thursday, two US Air Force B-1B bombers flew through the South China Sea during a 10-hour mission.
"China always remains vigilant and keeps effective surveillance over the military activities of the related countries in the South China Sea," the Chinese Defense Ministry said.
The ministry added that China "will resolutely safeguard national sovereignty and security, and maintain regional peace and stability."
China and several US allies in the region — namely Japan, Vietnam and the Philippines — have competing claims to the maritime borders and responsibility areas in the South China and East China seas.
In January 2013, the Philippines formally initiated arbitration proceedings against China's claim on the Spratly Islands, also known as the Nansha Islands, with the the Permanent Court of Arbitration ruling out in 2016 that the islands were not an exclusive economic zone for China.
However, China refused to recognize the tribunal's ruling.