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* U.S.-China trade spat back in spotlight

* Apple falls after holding back forecast on virus uncertainty

* Amazon.com slides as it sees possible Q2 loss

* Futures down: Dow 1.88%, S&P 500 2.08%, Nasdaq 2.30% (Adds more comments, updates prices throughout)

By C Nivedita and Shreyashi Sanyal

May 1 (Reuters) - U.S. stock indexes were set to fall at the open on Friday after President Donald Trump threatened to slap new tariffs on China over the coronavirus crisis, while Apple and Amazon became the latest companies to warn of more pain in the future.

Trump said late on Thursday his trade deal with China was now of secondary importance to the pandemic, as his administration crafted retaliatory measures over the outbreak.

The threat pulled attention back to the trade war between the world's two largest economies that has kept global financial markets on tenterhooks for nearly two years.

"(Trump) has really thrown a spanner in the works of the recovery that we've seen (in April)," said David Madden, a market analyst at CMC Markets UK in London.

"Traders are now afraid that we could now go back to the prolonged period of a trade spat between the United States and China."

Wall Street ended lower on the last day of April, taking some shine off the S&P 500 index's best month in 33 years that was driven by dramatic stimulus measures and hopes of reopening the economy from crushing virus-induced restrictions.

Technology heavyweight Apple Inc slipped 2.3% in premarket trading after Chief Executive Officer Tim Cook said it was impossible to forecast overall results for the current quarter because of uncertainty created by the virus.

Amazon.com Inc tumbled 4.8% after the company said it could post its first quarterly loss in five years as it was spending at least $4 billion in response to the coronavirus pandemic.

With nearly half of the S&P 500 companies having reported results so far, analysts expect a 14.4% fall in profits for the first-quarter and foresee an even sharper decline of nearly 37% for the current-quarter.

Exxon Mobil fell 1.4% after it reported a first-quarter loss, compared with a year-earlier profit, hurt by crashing oil prices, while Chevron Corp dipped 0.9% after its quarterly revenue took a big hit.

At 8:58 a.m. ET, Dow e-minis were down 455 points, or 1.88%. S&P 500 e-minis were down 60.25 points, or 2.08% and Nasdaq 100 e-minis were down 206.75 points, or 2.3%.

Markets will also keep a close watch on the ISM's purchasing managers index (PMI) data, due at 10:00 a.m. ET.

The report comes a day after data showed U.S. initial jobless claims totalled 3.84 million for the week ended April 25. "Every time we get economic data it adds a small amount of visibility into the significant unknowns about what to expect over the next 12 months," said Phil Toews, chief executive officer of investment firm Toews Corp in New York.

United Airlines Holdings Inc slipped 4.7% after posting a first-quarter loss of $1.7 billion.

Clorox Co rose 3.5% after the company posted its biggest rise in quarterly sales in nearly a decade and raised its annual forecast, reaping the benefits of a boom in demand for bleach and other disinfectants. (Reporting by C Nivedita and Shreyashi Sanyal in Bengaluru; Editing by Anil D'Silva)