President Barack Obama says he is to send up to 250 more special forces troops to Syria to support local militias in the fight against so-called Islamic State (IS).
The new deployment will bring to 300 the number of US special forces soldiers in Syria.
Speaking in Hannover, Germany, Mr Obama said the troops would not lead the fight but would provide training.
He has repeatedly ruled out sending in American ground forces.
"Make no mistake," he said. "These terrorists will learn the same lessons that others before them have, which is: your hatred is no match for our nations, united in defence of our way of life."
Earlier, US Deputy National Security Advisor Ben Rhodes said IS had been pushed out of some strongholds in northern and eastern Syria.
"We believe the commitment of additional US special forces can play a critical role," he added.
The goal, officials said earlier, was to encourage more Sunni Arabs to join Kurdish fighters in north-east Syria.