China has lodged a solemn complaint with the US over a US surveillance aircraft that flew over China's Nansha Islands in the South China Sea, with Chinese foreign ministry spokeswoman Hua Chunying saying that the close reconnaissance is highly likely to cause miscalculation and untoward incidents in the waters and airspace on Monday.

A US P-8A surveillance aircraft flew over islets under construction in the Nansha Islands on May 20 and the Chinese navy repeatedly asked the aircraft to leave via radio.

US Assistant Secretary of State Daniel Russel told reporters that the US flight was "entirely appropriate" and that US forces and military aircraft would "continue to fully exercise" their right to operate in international waters and airspace.

Hua said that there have never been any problems concerning the freedom of navigation and over-flight in the South China Sea. "Nor will there be any in the future," she stressed.

She said that China has been safeguarding the freedom of navigation and over-flight, however, this freedom is not tantamount to the violation of international law by foreign military vessels and aircraft in defiance of the legitimate rights and interests - as well as the safety of over-flight and navigation -of other countries.

China's determination to safeguard sovereignty and territorial integrity is "as firm as a rock" and China urges the US to correct its errors, stay rational and cease irresponsible comments and deeds, Hua said.

Despite China's firm stance, Philippine President Benigno Aquino said on Monday that Philippine aircraft will keep flying over disputed areas in the South China Sea.

Zhu Feng, director of the Collaborative Innovation Center of South China Sea Studies at Nanjing University, said that the US has been stepping up its activities in the South China Sea region and introduced new uncertainties to the regional security order.

He said that the US and the Philippines have been working together because the Philippine's complaints will provide "more legitimate" reasons for the US to interfere in South China Sea issues.

Manila has strengthened its alliance with Washington since 2011, but the risks of over-relying on another country has worried some Filipino politicians as they are concerned that the country may be sacrificed if China and the US conciliate on the South China Sea issue, said Zhu.