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President-elect Donald Trump’s nominee for secretary of state, Rex Tillerson, took a swipe at China during his confirmation hearing with the Senate on Wednesday.

The former Exxon-Mobil CEO likened China’s island-building in the South China Sea to the annexation of Crimea by Russia – a sharp change in tone from the Obama administration’s focus on cooperation.

Tillerson accused Beijing of “declaring control of territories that are not rightfully China’s.” He added that China’s actions were “extremely worrisome” and the U.S. failure to respond “has allowed them to keep pushing the envelope” in seas that carry $5 trillion of trade annually.

Beijing has reclaimed more than 3,000 acres of land and constructed military-grade infrastructure. Over the last year, has been fortifying its islands with surface-to-air missiles and fighter jet deployment. China has also flown its nuclear-capable bomber over the sea in an effort to intimidate the other countries that have territorial claims in the region.

"This is a threat to the entire global economy if China is allowed to somehow dictate the terms of passage through these waters," Tillerson told the Senate Foreign Relations Committee.

Under Obama, the U.S. has worked with China to fight climate change and contain Iran’s nuclear program. But Beijing has only half-heartedly pressed North Korea over its nuclear weapons program and has willfully disregarded Washington's appeals to moderate its activities in the South China Sea.

Asked if he supported a more aggressive U.S. posture, he said, "You'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."

Past U.S. administrations have entered office seeking to get tougher on China, and failed. Trump himself has threatened to impose punitive tariffs to address America's trade imbalance with China and tested ties by questioning the longstanding U.S. policy on the status of Taiwan.

A perennial challenge is to break China's long-time partnership with North Korea's hereditary dictatorship. Beijing is unwilling to exercise economic pressure that could destabilize its unpredictable ally, even as it shares U.S. concerns about the isolated nation's rapid development of nuclear weapons and ballistic missiles to deliver them.

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Earlier this week, outgoing Secretary of State John Kerry chided Beijing over its efforts to pressure North Korea, which relies on China for 90 percent of its international trade. He said China needed to "increase its focus" and that the U.S. may need "more forceful ways" of dealing with North Korea.

Tillerson spoke in starker terms. He accused China of making "empty promises" on North Korea and supported U.S. sanctions on Chinese companies found to be violating U.N. Security Council resolutions, recently beefed up to tighten restrictions on North Korean coal imports.

"If China is not going to comply with those U.N. sanctions then it's appropriate for the United States to consider actions to compel them to comply," Tillerson said.

Speaking generally, he described military force as the "least attractive option." And he sought to play down Trump's recent tweet on North Korea's development of a missile that could strike America — "It won't happen!" — saying it wasn't a "red line."

The Associated Press contributed to this report.