BEIJING, Oct. 27 -- A U.S. warship's foray close to islands claimed by China in the South China Sea on Tuesday was "a show of military force intended to militarize" the sea, according to Chinese Defense Ministry spokesman Yang Yujun.

China and other countries in the region are negotiating on a code of conduct concerning the South China Sea, and all parties have expressed willingness to commonly safeguard peace in the region, Yang said. In this context, the United States, a party not involved in the local disputes over maritime sovereignty, "sent a warship there to threaten China's national security, severely undermining regional stability."

USS Lassen entered waters near Zhubi Reef without the permission of the Chinese government, which the spokesman said posed a serious threat to China's national security, especially that of the Chinese facilities and staff on the reefs.

Yang denied that China's activity in the South China Sea will restrict freedom of navigation and overflight, saying that many vessels of many nationalities sail through the sea every day.

"The U.S. side has for many years proposed measures to ensure navigation safety of ships and aircraft of all parties, but its warship's operations today resulted in a close encounter between Chinese and U.S. troops, which threatened the safety of staff from both sides and is likely to trigger unexpected incidents. The United States has been very irresponsible," he said.

Its actions have weakened trust and ran counter to the consensus the two global giants have built on fostering a new model of major-country relations, according to the spokesman.

"The U.S. side should respect China's concerns, and solve divergences based on the principles of no conflict, no confrontation, mutual respect and win-win cooperation," Yang urged.

"We will take any measures necessary to safeguard our security," he warned.