(Reuters) - U.S. stock index futures fell on Friday after China increased its tariffs on U.S. imports to 125%, escalating a trade war that could disrupt global trade and trigger an economic downturn.

China's move comes after U.S. President Donald Trump on Thursday ramped up pressure on the country by lifting tariffs to 145%, even as he announced a 90-day tariff reprieve on most trading partners.

Stocks have been on a roller-coaster ride in response to tariff announcements in the past few days. Wall Street fell for four straight sessions, before bouncing back on Wednesday with the S&P 500 seeing its largest one-day percentage jump since October 2008.

Stocks, however, slumped again on Thursday and were more than 7% off from levels seen before last week, when Trump's "reciprocal" tariffs sparked the market rout.

Amid the volatility, all three indexes are set for robust weekly gains, with the Nasdaq set for its best weekly showing of the year so far.

At 04:32 a.m. ET, Dow E-minis were down 108 points, or 0.27%, S&P 500 E-minis were down 14.75 points, or 0.28% and Nasdaq 100 E-minis were down 54 points, or 0.29%.

Most megacap and growth stocks ticked lower in premarket trading, though Tesla lagged with a 1.9% fall.

Later in the day, U.S. quarterly earnings season will pick up pace with big banks such as JPMorgan Chase & Co, Morgan Stanley, Wells Fargo & Co and asset manager BlackRock scheduled to report before markets open.

(Reporting by Shashwat Chauhan in Bengaluru; Editing by Saumyadeb Chakrabarty)