BEIJING (AP) — The United States (US) Navy and Marine Corps are holding joint exercises in the South China Sea at a time of heightened tensions with Beijing over the shooting down of a suspected Chinese spy balloon.

The 7th Fleet based in Japan said Sunday that the USS Nimitz aircraft carrier strike group and the 13th Marine Expeditionary Unit have been conducting “integrated expeditionary strike force operations” in the South China Sea.

It said exercises involving ships, ground forces and aircraft took place Saturday but gave no details on when they began or whether they had ended.

China claims virtually the entire South China Sea and strongly objects to military activity by other nations in the contested waterway through which $5 trillion in goods are shipped every year.

The US takes no official position on sovereignty in the South China Sea, but maintains that freedom of navigation and overflight must be preserved. Several times a year, it sends ships sailing past fortified Chinese outposts in the Spratly Islands, prompting furious protests from Beijing.

The US has also been strengthening its defence alliance with the Philippines, which has faced encroachment on islands and fisheries by the Chinese coast guard and nominally civilian but government-backed fleets.

The US military exercises were planned in advance. They come as already tense relations between Washington and Beijing have been exacerbated by a diplomatic row sparked by the balloon, which was shot down last weekend in US airspace off the coast of South Carolina.

The US said the unmanned balloon was equipped to detect and collect intelligence signals, but Beijing insists it was a weather research airship that had accidently blown off course.

The incident prompted Secretary of State Antony Blinken to abruptly cancel a high-stakes trip to Beijing last weekend aimed at easing tensions.

After first issuing a highly rare expression of regret over the incident, China has toughened its rhetoric, calling the US move an overreaction and a violation of international norms. China’s defence minister refused to take a phone call from US Defense Secretary Lloyd Austin to discuss the matter.

The United States has since blacklisted six Chinese entities it said were linked to Beijing’s aerospace programs as part of its response to the incident. The House of Representatives also voted unanimously to condemn China for a “brazen violation” of US sovereignty and efforts to “deceive the international community through false claims about its intelligence collection campaigns.”.

The balloon was part of a large surveillance program that China has been conducting for several years, the Pentagon said. The US said Chinese balloons have flown over dozens of countries across five continents in recent years, and it learned more about the balloon program after closely monitoring the one shot down near South Carolina.

In its news release, the seventh Fleet said the joint operation had “established a powerful presence in the region, which supports peace and stability.”

“As a ready response force, we underpin a broad spectrum of missions including landing Marines ashore, humanitarian disaster relief, and deterring potential adversaries through visible and present combat power,” the release said.