The Pentagon had previously put the number of US forces in Afghanistan at about 11,000 but Trump in August authorized an increase requested by the commander on the ground, General John Nicholson, AFP wrote.
"We've just completed a force flow into Afghanistan," Joint Staff Director Lieutenant General Kenneth McKenzie told Pentagon reporters.
"The new number for Afghanistan is now approximately 14,000. Might be a little above that, might be a little below that as we flex according to the mission."
Nicholson has said he needs nearly 16,000 troops overall in Afghanistan, and NATO nations have pledged to help make up the difference.
Aside from additional troops, Trump's plan also comprises an open-ended US troop presence in Afghanistan, where his predecessor Barack Obama had ordered a calendar-based drawdown of American forces.