Chinese Foreign Ministry Spokesperson Hua Chunying.

Beijing said on Tuesday that it has expressed concern to Washington after two United States warships sailed through the Taiwan Straits.

Foreign Ministry spokeswoman Hua Chunying said at a daily news briefing in Beijing that China had closely followed the warships’ passage and monitored the entire process.

Hua said the Taiwan question concerns the sovereignty and territorial integrity of China and is the most important and sensitive issue in the Sino-US relationship.

“We urge the US side to strictly abide by the three China-US joint communiques and properly handle Taiwan-related issues so as to avoid impairing the bilateral relationship as well as peace and stability across the Taiwan Straits,” the spokeswoman said.

Pentagon spokesman Rob Manning said that the USS Curtis Wilbur and the USS Antietam sailed in international waters in what he calls a routine transit on Monday.

The US Navy conducted a similar mission in July, which was the first such voyage in about a year. The latest operation shows the US Navy is increasing the pace of passages in the area.

The fresh passage came days after a meeting between State Councilor and Defense Minister Wei Fenghe and US Secretary of Defense James Mattis on the sidelines of an ASEAN-led security forum in Singapore. Mattis said that the overall US policy toward Taiwan was unchanged.

Li Haidong, a US studies researcher at China Foreign Affairs University, said that such a move demonstrated that the US is trying to pressure China into making concessions on other issues by making use of the Taiwan question, which damages the political basis of bilateral ties.

Such high-profile missions are provocative, Li said.

Reuters contributed to this story.