U.S. President Donald Trump's decision to cut U.S. troop levels in Germany blindsided a number of senior national security officials, according to five sources familiar with the matter, and the Pentagon had yet to receive a formal order to carry it out, Reuters has learned.

Trump decided to remove 9,500 troops from Germany, one of America's strongest allies, reducing the number there to 25,000 from 34,500, a senior U.S. official said on Friday.

That official said it was the result of months of work by the U.S. military leadership and had nothing to do with tensions between Trump and German Chancellor Angela Merkel, who thwarted his plan to host an in-person Group of Seven (G7) summit this month.