US Secretary of State John Kerry says he is concerned by the pace and scope of China's reclamation projects in contested areas of the South China Sea.
He was speaking at a press conference with Chinese Foreign Minister Wang Yi in Beijing.
China claims almost the whole of the South China Sea, resulting in overlapping claims with Brunei, Taiwan, the Philippines, Vietnam and Malaysia.
Mr Wang said that China was determined to safeguard its sovereignty.
Mr Kerry is due to hold a series of meetings with Chinese President Xi Jinping and other top government officials in the Chinese capital.
Speaking after his meeting with Mr Wang, Mr Kerry said he "urged China... to take actions that will join with everybody in helping to reduce tensions and increase the prospect of a diplomatic solution".
Mr Wang replied that "the determination of the Chinese side to safeguard our own sovereignty and territorial integrity is as firm as a rock and it is unshakeable".
He described the development work in the South China Sea as something that fell "within the scope of China's sovereignty".
But he also said that China and the US had "more common interests than differences", urging both sides "to act in the spirit of mutual respect, seeking common ground while shelving differences".
US officials say China has reclaimed about 810 hectares of land (2,000 acres) of land in the disputed Spratly Islands since 2014.
Mr Kerry's visit comes as the Indian Prime Minister Narendra Modi wraps up his visit to China.
The two sides have signed trade and economic co-operation deals worth $22bn ($14bn) in Shanghai.
On Friday, Mr Modi and China's Premier Li Keqiang said they agreed to seek a "fair resolution" to border disputes arising from a boundary disagreement.