A Chinese aircraft carrier and around two dozen other Chinese warships were gathering in the western Pacific, according to authorities in Taiwan and Japan, an unusually large group suggesting Beijing may be planning major naval exercises.

The movement follows a flurry of U.S. military activity in the region, and comes as efforts by Washington and Beijing to improve relations appear to have lost momentum.

China has made no announcement about coming naval drills, but it has condemned a transit of the Taiwan Strait by a U.S. destroyer and Canadian frigate on Saturday. The U.S. held other naval exercises near China involving allies such as Japan and the Philippines in recent weeks.

The Chinese aircraft carrier Shandong sailed toward the western Pacific Ocean to the southeast of Taiwan on Monday for training, Taiwan’s military said. It also said it identified 20 Chinese warships around Taiwan, without giving further details.

Japan’s military said six Chinese destroyers and two Chinese frigates were spotted on Monday morning close to Japan and monitored as they sailed southeast between the Japanese islands of Okinawa and Miyako into the Pacific Ocean.

“The Shandong undoubtedly poses a new threat” to Taiwan, Gen. Huang Wen-chi, the assistant deputy chief of general staff for intelligence for Taiwan’s military, said at a Tuesday news conference in Taipei, noting that the aircraft carrier has been paired with new large destroyers.

On Saturday, the guided-missile destroyer USS Ralph Johnson and Royal Canadian Navy frigate HMCS Ottawa sailed through the Taiwan Strait, according to the U.S. military, in a so-called freedom-of-navigation operation.

China’s military, the People’s Liberation Army, scrambled air and naval forces to monitor the U.S. and Canadian ships, according to a spokesman for the PLA’s Eastern Theater Command.

“Troops in the theater remain on high alert at all times, resolutely safeguarding national sovereignty and security as well as regional peace and stability,” the spokesman said.

China views transits by American and other warships of the Taiwan Strait, a channel about 100 miles wide that separates Taiwan from the Chinese mainland, as particularly provocative. In June, the U.S. accused a Chinese destroyer of sailing dangerously close to a U.S. warship as it traveled through the waterway.

The USS Ralph Johnson also conducted a joint exercise with a Philippine navy ship in the South China Sea last week, according to the U.S. military. In August, the U.S., Japan, Australia and the Philippines held joint naval exercises in the South China Sea, a region that China claims broad ownership of.