GENEVA (AP) — China and the United States clashed again over their respective trade policies Monday at a time the two countries are trying to iron out their differences so further U.S. tariffs are not imposed on Chinese goods.

Dennis Shea, the U.S. ambassador to the WTO, said criticism from China about the U.S.'s "unilateralist and protectionist" approach to trade was unwarranted.

He also insisted the U.S. wants to reform the global trading system to make it fairer for U.S. citizens and defended America's long role in supporting that system for seven decades, at the World Trade Organization and its predecessors.

"The United States is raising serious concerns with the functioning and direction of this important institution, and the fundamental challenge posed by China's state-led, mercantilist approach to the economy and trade," Shea said during closed-door remarks for the WTO's 14th and latest regular "trade policy review" of the United States.

He said the global trade environment was "heavily skewed" in favor of China.

The U.S. and China are locked in a trade standoff, though President Donald Trump agreed this month to postpone more U.S. tariff hikes on Chinese goods for 90 days while negotiations continue.

Shea's Chinese counterpart at the WTO drew upon lessons from Spider-Man to remind the U.S. of the need to be responsible in trade affairs: "'With great power comes great responsibility.' And Spider-Man certainly lived up to that."

Zhang Xiangchen upbraided the U.S. over tariffs and blocking appointments to the WTO's appeals body, which could stop working by December next year because a term expiration would reduce its membership below the minimum of three people.

He also said Trump's tariff increases on steel and aluminum products were "based on dubious national security concerns" and blasted U.S. efforts to put WTO's appeals body "in paralysis."

"Whether it is a small family or an international organization, a top dog should act like a top dog," Zhang said, in an apparent allusion to the United States. "It cannot only see a narrow spectrum of its own self-interest, and it certainly should not do whatever it wishes at the sacrifice of the others."

Marc Vanheukelen, the European Union's ambassador to the WTO, noted how in 2016, at the last review of the U.S., he had hoped that President-elect Trump's "protectionist rhetoric would end" after he took office.

"Today, unfortunately, rhetoric has turned into reality and the repercussions of tariffs and other restrictions are being felt at the heart of this organization, and more generally in global growth prospects," he said.

"The multilateral trading system is in a deep crisis and the United States is at its epicenter for a number of reasons."