The South China Sea may the first flashpoint for incoming Trump administration, at least if his designated Secretary of State Rex Tillerson, has his way.

At his conformation hearings last week, Tillerson was questioned about China’s military buildup on artificial islands it constructed in the key international and contensted waterway.

Tillerson responded that the US should block access to those islands, although he did not provide any details of what sort of operation that would involve. It’s not clear whether Tillerson was reflecting President-elect Donald Trump’s views on the issue.

The dispute over the South China Sea includes implications for freedom of navigation and overflight in the region. An estimated $5 trillion in cargo passes through the South China Sea every year by ship. Brunei, Malaysia, the Philippines, Taiwan, and Vietnam have all also articulated claims to parts of the area in dispute. In July 2016, a United Nations arbitration panel in the Hague ruled in favor of the Philippines in a case brought against China over claims in the South China Sea.

“We’re going to have to send China a clear signal,” said Tillerson, “that first, the island-building stops and second, your access to those islands also is not going to be allowed.”

The Obama administration has spoken out strongly against China’s moves in te region and has pledged to ensure freedom of navigation in the waterway, sending navy ships to contested areas. Obama has not threatened to block access to the islands, however.

For its part China has acknowledged building what it calls defensive military facilities on the islands. A Chinese foreign ministry spokesman responded to Tillerson’s comments, saying China had the right to conduct “normal activities” in its own territory.

There have been incidents involving US and Chinese ships in the South China Sea. Last month, a Chinese ship seized a US navy underwater drone off the Philippines, but later returned it.

On other issues, said he favored maintaining U.S. sanctions against Russia, that he did not oppose the Trans-Pacific Partnership trade deal, and that Obama administration policies, such as opening trade with Cuba and the Iran nuclear deal, should be subject to a full review. He did not call for an outright rejection of the 2015 accord with Tehran. Tillerson reportedly opposed U.S. sanctions against Russia in 2014 over its annexation of Crimea saying they would be ineffective.