China on Monday said it has "serious concern" over Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's comments a day earlier about the president-elect's call with Taiwan's leader and the One China policy, according to multiple reports.

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"China expresses serious concern on this subject," Chinese Foreign Ministry spokesman Geng Shuang told reporters, according to NBC News.

He stressed "that the Taiwan question has a bearing on China's sovereignty and territorial integrity," according to CNN.

"Adhering to the One China principle is the political bedrock for the development [of] U.S.-China relations. If it is comprised or disrupted, the sound and steady growth of the bilateral relationship, as well as bilateral cooperation in major fields would be out of question," he added.

"We urge the new U.S. leader and government to fully understand the seriousness of the Taiwan issue, and to continue to stick to the one-China policy," Geng said. The Independent also reported early Monday that China sent a nuclear bomber on a flight over the South China Sea to "send a message" to Trump

The president-elect on Sunday said he fully understands the One China policy, before going on to question the agreement.

"I don't know why we have to be bound by a One China policy unless we make a deal with China having to do with other things, including trade," Trump said in an interview with "Fox News Sunday."

He said during the interview he doesn't want China "dictating" to him.

He also defended the controversial call he took earlier this month with Taiwan's president — a conversation that his top aides have downplayed as just a congratulatory call.

Trump faced backlash for accepting the call, which some said broke from the One China policy.

President Jimmy Carter formally declared the People's Republic of China the sole government of China in 1979, ending formal U.S. diplomatic relations with Taiwan. Beijing refuses to recognize any country that has formal diplomatic relations with Taiwan, which it considers a breakaway province.