– Global trade tensions escalated this week as the U.S. renewed its tariff war with China, sending major stock indexes tumbling as fears of an economic slowdown rattled investors around the world.

The S&P 500 index had its worst week of trading this year, as shares on exchanges from Tokyo to London fell Friday — one day after President Donald Trump announced new tariffs on another $300 billion worth of Chinese imports following stalled talks. Beijing's response was swift.

"China's position is very clear that if U.S. wishes to talk, then we will talk," Zhang Jun, China's new U.N. ambassador, said Friday. "If they want to fight, then we will fight."

As the two sides appeared to drift further from a deal, Japan and South Korea veered toward their own trade confrontation Friday, injecting greater uncertainty.

The disputes could exacerbate fears of a global economic slowdown and threaten to crimp the United States' economic expansion, its longest on record. The European Central Bank is preparing to try to bolster the eurozone economy to help weather the slowdown in growth, while China has experienced its slowest economic growth in 27 years. On Wednesday, the Federal Reserve cut interest rates for the first time in over a decade to get ahead of possible downturns.

New hiring data released Friday showed the U.S. economy continued to chug along, with employers adding 164,000 jobs in July. But there were signs of cooling, complicating matters as the trade war begins to reshape the economy in ways that could run counter to Trump's goals of strengthening it.

The Commerce Department announced Friday that the trade deficit with China fell in the first six months of the year compared with a year earlier, and the country fell from being the United States' largest trading partner to being its third, after Mexico and Canada. However, the overall trade deficit increased. U.S. exports of goods and services to the rest of the world were flat from the year before, while imports from the rest of the world grew.

In a campaign rally in Cincinnati on Thursday, the president was unbowed with his strategy.

"Until such time as there is a deal, we will be taxing the hell out of China," Trump told a cheering crowd.

Chinese officials and the state-run media have grown increasingly strident in response to Trump's tactics. "We definitely will take whatever necessary countermeasures to protect our fundamental right, and we also urge the United States to come back to the right track in finding the right solution through the right way," said Zhang.

After a series of missteps and misunderstandings, the U.S. and China appear to be nowhere close to a trade agreement at a time when trade barriers remain in place with other U.S. partners. Trump's rewrite of the North American Free Trade Agreement is still stalled in Congress, awaiting the support of Democrats. His threat of auto tariffs has not yet persuaded Japan or Europe to sign a trade deals with the U.S., as he intended. And the European Union, India, China, Turkey and others have responded to Trump's aggressive trade tactics by putting their own retaliatory tariffs on U.S. products.

If the president's newly threatened tariffs go into effect, the U.S. will have imposed levies on all of the goods it imports from China, which totaled $539.7 billion in 2018.

Trump said that the new tariffs go into effect on Sept. 1, leaving a window for the U.S. and China to try to work out their differences. But that appears to be a difficult task. Negotiators continue to disagree over how the agreement would be enshrined in China's laws, how many of Trump's tariffs on China would be removed, and how many U.S. goods China would purchase.