The Chinese navy asked the U.S. vessels to leave after they entered waters adjacent to Gaven and Chigua reefs in the Spratly Islands, which Beijing calls Nansha, the foreign ministry said.

The move comes on top of trade tensions as President Donald Trump said Sunday he would raise tariffs on $200 billion in Chinese goods on Friday because talks were moving "too slowly."

"The relevant actions of the U.S. warships violated China's sovereignty and undermined peace, security and good order in the relevant sea areas," ministry spokesman Geng Shuang said at a press briefing.

"The Chinese side expresses strong dissatisfaction and resolute opposition" to the operation, Geng added, noting that the ships had entered "without permission."

The U.S. Navy regularly conducts "freedom of navigation operations" to challenge Beijing's vast claims in the sea.

The People's Liberation Army identified the two U.S. ships as the USS Preble and the USS Chung-Hoon, both guided-missile destroyers.

Air and sea forces were deployed to identify and ask the ships to leave, a spokesman for the PLA's Southern Theatre Command said on China's Twitter-like Weibo platform.

"The (Southern) Theatre troops will maintain a high degree of alert and take all necessary measures to resolutely defend the sovereignty of the country and resolutely safeguard peace and stability in the South China Sea," said the spokesman, Li Huamin.

Beijing has built artificial islands and military installations in the sea, including on the Spratlys.

China claims nearly all of the sea, though Taiwan, the Philippines, Brunei, Malaysia all claim parts of it. The Philippines, Brunei, Malaysia, Indonesia and Taiwan have competing claims in the region. Vietnam, which calls the waters the East Sea, has repeatedly asserted its sovereignty over the waters.

"China has indisputable sovereignty over the South China Sea islands and its adjacent waters. No matter how provocative foreign warships are, this fact cannot be changed," Li said.

Geng said the situation in the sea was "constantly stabilising" due to the "joint efforts" of Beijing and the Association of Southeast Asian Nations.

"Under such circumstances, China urges the U.S. to stop such provocative acts, to respect China's sovereignty and security interests, and respect the efforts of regional countries to maintain peace and stability in the South China Sea," he said.

"At the same time, China will continue to take all necessary measures to safeguard national sovereignty and safeguard peace and stability in the South China Sea."