Rex Tillerson, the former CEO of oil giant ExxonMobil, told his confirmation hearing Wednesday that China's activity in the waters, which see $5 trillion in shipborne trade a year, was "extremely worrisome."
The comments during his day-long testimony
before the Senate Foreign Affairs Committee look set to inflame tensions with China, which claims much of the South China Sea as an integral part of its territory.
He also compared China's territorial claims to Russia's military intervention and the annexing of Crimea in 2014
. "Building islands and then putting military assets on those islands is akin to Russia's taking of Crimea. Its taking of territory that others lay claim to," Tillerson said.
"We're going to have to send China a clear signal that first, the island-building stops, and second, your access to those islands also not going to be allowed."
Tillerson added that the Obama administration had neglected to tackle the problem, saying the "failure of a response has allowed them to just keep pushing the envelope on this."
Analysts said Tillerson's testimony, combined with his future boss's earlier pronouncements, signaled that a Trump administration is poised to take a much tougher stance on China.
Since winning the election Trump has lashed out at China on Twitter, made clear he's serious about wringing a new trade deal from Beijing and upended US policy toward Taiwan -- an issue of deep sensitivity for China.
"All the quotes taken together do signal that, like Trump and some of his advisers, are poised to take much firmer stance on China in the South China Sea and across the board," said Ashley Townshend, a research fellow, at the United States Studies Centre at the University of Sydney.
But Townshend doubted whether Tillerson would really follow through on some of the specifics of his testimony.
"The US cannot block China's access to the islands without causing a confrontation, probably a military confrontation, and it would be illegal for the US to block access to the vast majority of those islands and reefs," he said.
In response to Tillerson's remarks, China's Ministry of Foreign Affairs spokesman Lu Kang appeared to say Thursday that the US should stay out of the dispute.
He said China and other claimants had returned to a "dual track" approach to try and "resolve the issue peacefully," saying that the situation in the South China Sea had "cooled down."
"We hope countries outside the region will respect this consensus that is in the interest of the region and the world."
Under Obama, the US has repeatedly called for freedom of navigation in the South China Sea and sailed and flown its assets near the islands China controls -- drawing repeated warnings from China.
China has reclaimed more than 3,000 acres of land in the Spratly Islands
since 2014, turning reefs and sandbars into islands equipped with airfields, ports and weapons.
Yvonne Chiu, an assistant professor of politics at Hong Kong University, said that militarily the US would be able to kick China off the islands but doubted this was something the Trump administration really had the stomach for.
"I think the ultimate goal is to make sure the islands aren't used to threaten other countries in the region," she said.
Tillerson, who said that he'd only spoken to the President-elect about world affairs in general terms, did appear to depart from the views of his future boss on North Korea, which Townshend said would be the biggest challenge for the new president.
"I think a lot of our troubles today are that we do not enforce -- we make commitments, we say we are going to do something and then we don't enforce it," Tillerson said.
"And that is again a mixed message that I think has been sent in the case of North Korea and our expectations of China. I think we have to be clear-eyed as to how far China will ago, and not get overly optimistic as to how far they'll go," Tillerson said.
On Taiwan, Tillerson didn't diverge from current US policy, saying he'd reaffirm the Taiwan Relations Act, which has guided ties between the two since 1979.