BEIJING—A U.S. blockade of Chinese-built islands in the South China Sea risks triggering a dangerous military confrontation and would be too costly to sustain long-term, experts warned Thursday after U.S. President-elect Donald Trump’s pick for secretary of state proposed such a move.
The proposal by Rex Tillerson in his Senate Foreign Relations Committee confirmation hearing Wednesday is likely to have alarmed Chinese leaders as it went well beyond Mr. Trump’s own remarks, as well as the advice from more hawkish elements of the U.S. military, security experts said.
China’s initial response was low key, with a foreign ministry spokesman declining to comment on the former Exxon Mobil Corp. chief executive’s suggestion, while defending Beijing’s actions within what it sees as its sovereign territory.
“Mr. Tillerson said the disagreements between the two countries shouldn’t exclude areas of cooperation, and I do agree with him on that,” the spokesman, Lu Kang, told a regular news briefing.
Several Chinese and Western experts saw Mr. Tillerson’s suggestion less as a concrete proposal than a signal to Senate hawks—and to Beijing—that the Trump administration will take a tougher stance on the South China Sea than its predecessor.
Even so, his proposal intensified the uncertainty that has engulfed U.S.-China relations in recent weeks following Mr. Trump’s pledges to confront Beijing on trade and territorial issues, including the Chinese island outposts he has described as a “massive fortress.”
Mr. Tillerson’s remarks “sound very alarming,” said Zhu Feng, an expert on the South China Sea at Nanjing University. “I think he wanted to outline the worst-case scenario. The problem is that it could pull both powers into a vicious cycle.”
China’s island-building in the past three years has raised concern in the U.S. and among its Asian allies and partners that Beijing plans to use the facilities to enforce its claims to almost all the South China Sea, one of the world’s busiest shipping routes.
The Obama administration has repeatedly criticized China’s actions and sent military ships and planes close to the islands, but has refrained from taking sides in the territorial dispute.
Mr. Tillerson told the Senate hearing that China’s construction of the islands and installation of weaponry there were “akin to Russia’s taking Crimea” from Ukraine in 2014, and that a weak U.S. response had encouraged Beijing to “push the envelope.”
Asked whether he supported a stronger U.S. response in the South China Sea, he 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 is also not going to be allowed.”
Mr. Tillerson’s remarks came on the eve of a visit by incumbent Secretary of State John Kerry to Vietnam, one of several governments whose claims overlap with China’s and with whom the U.S. has tried to forge closer defense ties.
Earlier on Wednesday, China issued a white paper on regional security in which it pledged to work with the new U.S. administration but warned it may “make the necessary response” to any infringement of its sovereignty.
To enforce a blockade, the U.S. would have to use force to prevent Chinese ships and planes reaching the islands in violation of Washington’s own commitment to freedom of navigation through international waters and airspace, several analysts said.
“That would be tantamount to an act of war,” said Richard Bitzinger, a security expert at Nanyang Technological University in Singapore. “How would the Chinese respond? With every tool at their command,” he said. “Washington has options, but trying to pull off a Cuban-missile-crisis style quarantine is not one of them.”
Beijing would be certain to resist any blockade, leading to a military confrontation that could quickly escalate, and although U.S. firepower is greater overall, sustaining such an operation close to China could inflict heavy costs, security experts said.
China’s arsenal includes antiship missiles, jet fighters and bombers that can reach the islands from the mainland, and Beijing could use coast guard ships and a maritime militia of thousands of commercial vessels to back up its navy. China could easily break such a blockade, said Shen Dingli, an international security expert at Shanghai’s Fudan University.
Mr. Tillerson didn’t say how the U.S. could enforce a blockade of the islands, which according to recent satellite images include three airstrips capable of handling jet fighters and have all been equipped with antiaircraft weapons.
China has largely completed land reclamation around the islands and in the near term will likely avoid installing more heavy weaponry that could provoke a stronger U.S. response, Chinese analysts said.
Beijing would be conscious of the political context for Mr. Tillerson’s remarks, the Chinese experts said. “He has to show he’ll have a tough attitude—only this way will he be smoothly confirmed,” said Jia Qingguo, professor of international studies at Peking University.
Relations between Beijing and Washington are in a transitional period as Mr. Trump has yet to complete his cabinet or formulate a coherent China policy.
Still, these experts say the Trump team’s consistently harsh comments on the South China Sea make it likely that his administration will take some kind of action there early on—most likely an escalation of “freedom of navigation” operations close to the Chinese-built islands in the Spratlys archipelago.
“The one option the U.S. has is to send task forces through the Spratly islands on a regular basis,” said Carlyle Thayer, professor emeritus at the Australian Defense Force Academy.
“For the U.S. to do what Tillerson says would be a major commitment on a par with the Cuban missile crisis,” he said.
—Te-Ping Chen in Beijing and Jake Maxwell Watts in Singapore contributed to this article.
Write to Jeremy Page at jeremy.page@wsj.com