The decision is partly driven by Russia's battlefield successes and concerns over a possible abrupt change in U.S. policy following Donald Trump's inauguration on Jan. 20. But such an expansion of U.S.

The aim is to expand the pool of fighting-age men available to help a badly outnumbered Ukraine in its nearly three-year-old war with Russia.

More than 1 million Ukrainians are currently serving in military units, but the US believes it needs more than 160,000 additional troops to keep up with Russia.