China sent out a stern message to the United States after an American B52 bomber flew close to the Spratly islands last week, that China claims is its territory in the South China Sea.

US officials said that the bomber had 'unintentionally' come within two nautical miles of an artificial island built by China in the South China Sea. 

The incident reportedly occurred on 10 December, and China claims that two B52 bombers had entered the airspace near the reef. 

China said that such incidents harm peace in the region and registered a formal protest through the US embassy. 

"US's behaviors are serious military provocations that harm regional peace and stability. China urges US to prevent such hazardous actions, saying it will take all necessary measures to defend national sovereignty and security," the Chinese Defence Ministry said, according to CCTV News. 

Pentagon officials said that they are investigating the case as 'there was no intention of flying to within 12 nautical miles', The Wall Street Journal reported. 

The incident is likely to raise tensions between the two superpowers as it comes just two months after a US warship came within 12 nautical miles of Subi Reef in the Spratly Islands in October. 

Last month, the US Navy said that it will conduct another such patrol by the end of the year under 'freedom of navigation operations'. 

US has repeatedly disputed China's claims over the Spratly islands in the region, which is also contested by neighbouring nations such a Philippines and Vietnam.

The Chinese warning comes days after it had expressed condemnation of US' deal to sell arms and warships to Taiwan, which China considers a renegade province.