China urged the US to stop provocative actions following the flight of two B-52 bombers over the area near China's Nansha islands.
"China takes the incident seriously and has lodged solemn representation with the US," Xinhua quoted Foreign Ministry spokesperson Hong Lei as saying on Saturday.
Two US bombers flew into the airspace near an island and reef of the Nansha Islands early on December 10.
Pentagon said the flight might have strayed off course due to weather and an investigation had been launched.
Hong said Chinese Defence Ministry responded to the incident early on Saturday. The Chinese military kept strict surveillance and remained on high alert for US bombers' flight on December 10, and warned and expelled the bombers, he said.
According to Hong, the US said it would investigate the incident.
It must be stressed that China has always respected and safeguarded every country's flight and navigation freedom around the world in line with international laws, Hong said.
"But we never allow anyone to infringe upon China's sovereignty and undermine China's security interests with the excuse of flight and navigation freedom," the spokesperson said.
"We once again urge the United States to seriously reflect upon and correct its mistakes, take effective measures to prevent similar dangerous and provocative actions, and stop any move that may undermine China's sovereignty and security interests and affect the peace and stability of South China Sea," Hong said.
