China has warned the U.S. to withdraw sanctions on its military or face consequences. The U.S. imposed the sanctions on Thursday over China's purchase of Russian fighter jets and surface-to-air missile equipment.

Foreign Ministry spokesman Geng Shuang said China "is strongly outraged by this unreasonable action by the U.S. We strongly urge the U.S. to immediately correct its mistakes and revoke the so-called sanctions. Otherwise, it must take all the consequences."

Russia sold Sukhoi Su-35 fighter jets and a S-400 surface-to-air missile system to China, which has not joined U.S. sanctions against Russia for its actions in Ukraine and interference in the U.S. political system.

On Thursday Secretary of State Mike Pompeo, in consultation with Treasury Secretary Stephen Mnuchin, imposed sanctions on the Chinese military's Equipment Development Department, which is responsible for improving military technology, and its head, Li Shangfu, and added them to a list of people whose assets in the U.S. are frozen. Americans are also "generally prohibited" from doing business with them.

The U.S. says China's purchase of the weapons from Russia violated a 2017 law, the Countering America's Adversaries Through Sanctions Act.

Russia has criticized the U.S. sanctions on China. Deputy Foreign Minister Sergei Ryabkov said the U.S. was "thoughtlessly undermining" global stability, and warned it against "playing with fire."

The sale of equipment follows massive military exercises in Russia which involved contingents from China and Mongolia. Trade between China and Russia, which share a border to China's north, has also increased in recent years, climbing to $87 billion in 2017, from $64.2 billion in 2015, according to Chinese and Russian state media.

Russian President Vladimir Putin said this week he expects trade between the two countries to top $100 billion in 2018.

Zach Witlin, an analyst with the Eurasia Group, told NPR that ties between Russia and China will likely continue to strengthen:

"Chinese Russian cooperation is in an upswing, and it probably is a long-term phenomenon. This owes itself to several reasons.

"One of them, is in general, Russia's geopolitical relationship with the West, the United States especially, but also Europe, is extraordinarily strained, and that has given a premium towards cooperating with non-Western powers, of which China is probably the leading example around the world.

"China in its own right is slowly but surely asserting itself in a greater way on the world stage, and sees greater reasons to value its cooperation with Russia."

Witlin says these deepening ties have not made up for the economic toll of U.S. sanctions on Russia.

"With Russia, there was perhaps an excessive expectation after sanctions first came into effect following the Ukraine crisis and annexation of Crimea, that China would be able to substitute in some way, shape, or form, at least to a certain minority percentage, the lost investment and economic potential from western sanctions, and that really has not materialized," Witlin said. "The economic deals that have gone forward between Russia and China, have been more selective than what the Russian side really wanted. ... So it may be a long-term relationship that's growing in cooperation, but there is a limit to the depth in how far that cooperation is willing to go."

Putin and China's President Xi Jinping met earlier this month at the Eastern Economic Forum in Russia. "The president and I agree that since the beginning of this year, Russian-Chinese relations have been showing dynamic growth," Xi said, according to news reports.

Dmitri Trenin, the director of the Carnegie Moscow Center, wrote an op-ed for China Daily, China's state-run newspaper, last week. "Putin and Xi are on the same page when it comes to the fundamental concept of a desirable world order," Trenin wrote. China and Russia's shared goals include "several independent power centers instead of a single-nation hegemony; protection of state sovereignty from foreign political and ideological influence; and full equality in relations among the major powers, including the United States."

China and Russia's increasing military and economic ties come as a trade war escalates between China and the U.S.

The U.S. has imposed tariffs on about 40 percent of China's exports to the U.S., which are worth hundreds of billions of dollars a year. In return, China has retaliated against U.S. exports.

President Trump has threatened further tariffs. This week, President Trump wrote on Twitter, "China has been taking advantage of the United States on Trade for many years. They also know that I am the one that knows how to stop it. There will be great and fast economic retaliation against China if our farmers, ranchers and/or industrial workers are targeted!"

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