The United States is not qualified to make "carping comments on China" as it has not signed on to the United Nations Convention on the Law of the Sea, the Foreign Ministry said on Friday.

The comment came as top diplomats from China and Russia urged countries outside the South China Sea region not to stir up tension there.

On Thursday, US Deputy Secretary of State Antony Blinken told a House of Representatives hearing that China "can't have it both ways," by being a party to the convention but rejecting its provisions, including "the binding nature of any arbitration decision", Reuters reported.

Blinken was challenging China's rejection of arbitration by a tribunal at the Permanent Court of Arbitration in The Hague, which was set up in 2014 following a unilateral request by the Philippines.

Foreign Ministry spokeswoman Hua Chunying told a regular news conference on Friday that the US has not signed on to the convention.

Instead, Washington had introduced its so-called Freedom of Navigation Program in 1979 ahead of the convention's introduction in 1982, Hua said.

Hua also said that it is "a secret known to all" that the US subscribes to international law when it is favorable, and ignores the law when it is not.

She said more than 30 countries, in addition to China, have filed declarations in accordance with the convention to exclude disputes about maritime delimitation from compulsory arbitration and other compulsory dispute settlement procedures.

"China's rejection of the arbitration and refusal to be part of it is to ensure the solemnity of international laws, including the convention, and to oppose abuse of them," Hua said.

Ruan Zongze, vice-president of the China Institute of International Studies, said that behind the current maritime tension lies the "zero-sum thinking" and US alignment with allies that seek greater engagement in the South China Sea issue.

Meanwhile, China and Russia have "achieved important consensus on the South China Sea issue", according to a release by the Chinese Foreign Ministry on Friday.

They agreed that the disputes in the South China Sea "should be peacefully resolved on the basis of historical facts and international law, and through consultation and negotiation by countries directly concerned", the release said.