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.
While in Hannover, President Obama is discussing Syria and other foreign policy issues with leaders of the UK, Germany, France and Italy.
On Sunday, Mr Obama said he was "deeply concerned" about a surge in violence in Syria, with the opposition accusing the government of violating a truce brokered by the US and Russia.
Mr Obama has resisted calls to send combat troops into Syria, where a five-year-old conflict has killed more than 250,000 people and displaced some 11 million others.
Of those, four million have fled abroad, including growing numbers who are making the dangerous journey to Europe.
A concerted attack on IS: analysis by Jonathan Marcus, BBC defence and diplomatic correspondent
Last October, the US deployed up to 50 "special operators" to northern Syria to help train and coordinate anti-IS forces there. Their focus seems to be on trying to get more Syrian Arab forces into the field as part of a loose alliance of Arab, Kurdish and other fighters.
Some 250 more troops, many of them special forces, will build upon and expand this work. The US sees this as reinforcing success. IS has suffered several setbacks in both Iraq and Syria and it is under concerted air and cyber attack, intended not just to push its forces back but to destroy its sources of revenue - its oil and physical stocks of money.
Bolstering Arab forces also helps to placate the Turks, alarmed by the military success of Kurdish fighters in Syria.
Safe zone call
The crisis has put pressure on EU states which are struggling to halt a massive influx of migrants and refugees.
Germany received close to 477,000 asylum applications last year as almost 1.1m migrants arrived. A further 181,405 asylum applications have been made so far this year.
Speaking alongside Mr Obama on Sunday, German Chancellor Angela Merkel urged warring parties to set up safe zones in Syria where refugees would be protected within the country.
She expressed hope that such a plan might eventually be agreed at peace talks taking place in Geneva.
Mr Obama, however, said it would be "very difficult" for those zones to work without a large military commitment.
IS has lost parts of the territory it once controlled in Syria. Most recently, they were pushed back by Russian-backed Syrian forces from the strategic city of Palmyra.
The group has also had significant setbacks in Iraq, including the loss of Ramadi, the capital of Anbar province.
The US has led a coalition against the militant group in both Syria and Iraq.
Also in his speech in Hannover, President Obama repeated calls for Europe to remain united, two months ahead of a vote in the UK over whether to leave the EU.
"If a unified, peaceful, liberal, pluralistic, free-market Europe begins to doubt itself... then we can't expect the progress that is just now taking hold in many places around the world will continue," he said. "Instead, we will be empowering those who argue that democracy can't work."