Defense Secretary Jim Mattis has canceled a planned visit to China this month, a sign of the rising tensions between Washington and Beijing over trade and China’s expansive maritime claims in the South China Sea.

The word from the Pentagon comes one day after a U.S. warship had an “unsafe” encounter with a Chinese destroyer during a freedom-of-navigation operation near the disputed Spratly Islands in the South China Sea.

Mattis was scheduled to visit Beijing in mid-October, but dropped the stop from his itinerary when China failed to commit to any meetings with a Chinese official of comparable stature, a defense official said.

The official said the decision by Mattis to cancel his visit was not directly related to the incident in the South China Sea, in which the Chinese ship was said to have come as close as 45 feet to the destroyer USS Decatur as it was sailing in international waters 12 nautical miles off of the Gaven and Johnson reefs.

Nevertheless, relations between the U.S. and China are strained over a number of issues, including two recent U.S. B-52 bomber flights over the same general area to assert the right to fly in international airspace.

The bombers, based in Guam, “participated in a routine training mission in the vicinity of the South China Sea and Indian Ocean,” according to a statement from Pacific Air Forces.

“These recent missions are consistent with international law and United States’ long-standing and well-known freedom of navigation policies,” the statement said.

In response, China denied permission for the amphibious assault ship USS Wasp to make a port call in Hong Kong this month.