U.S. Air Force officials made it clear to North Korea they were not messing around, launching a surprise military exercise with full combat air power in Japan Wednesday.

The jets in the arsenal of the 18th Wing, which conducted the exercise, include HH-60 Pave Hawks, F-15 Eagles, E-3 Sentries and KC-135 Stratotankers. Military officials call it the Air Force's largest combat-ready wing.

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Photos showed a large group of the jets on a runway at Kadena Air Base in Japan.

Meanwhile, China has urged North Korea's opponents not to do anything rash, Reuters reported, despite signs the rogue nation soon may conduct a sixth nuclear test.

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"Military force cannot resolve the issue," Chinese Foreign Minister Wang Yi said in Beijing. "Amid challenge there is opportunity. Amid tensions we will also find a kind of opportunity to return to talks."

Satellite imagery showed activity at North Korea's Punggye-ri testing site, a sign the secretive regime could be preparing for another nuclear test, analysts from the U.S. research institute 38 North wrote.

On Wednesday, President Trump voiced confidence Chinese President Xi Jinping would help him control North Korea's mounting threat.

"I think he wants to help us with North Korea," Trump said of Xi, crediting China in a White House news conference with taking a "big step" by turning back boats of coal that North Korea sells to its northern neighbor. North Korea conducts some 90 percent of its trade with China.

Trump also repeated that trade concessions could be on the table for more cooperation on North Korea. He said he told Xi last week: "The way you're going to make a good trade deal is to help us with North Korea, otherwise we're just going to go it alone. That will be all right, too. But going it alone means going it with lots of other nations."'

The United States has been urging Beijing to use its economic leverage with North Korea, which conducted two underground nuclear explosions and two dozen missile tests last year. It is moving closer to developing a nuclear-tipped missile that could threaten the U.S. mainland, analysts have said.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.