Barack Obama has announced the deployment of an extra 250 US special operations forces to Syria to assist troops trying to dislodge Islamic State extremists from the war-torn country.
The US president, hailing recent gains against the group, said the added troops would help "to keep up this momentum" against IS.
The move will significantly broaden the American presence in Syria, bringing the number of personnel to roughly 300, up from about 50 special operations forces currently there.
Mr Obama revealed his decision a week after Defence Secretary Ash Carter announced that more than 200 US troops will soon be headed to Iraq, where local forces are also battling Islamic State militants who control areas of that country.
He said the newest insertion of US forces will not be in combat roles.
"They're not going to be leading the fight on the ground, but they will be essential in providing the training and assisting local forces," Mr Obama said during a speech in Hanover, Germany, that capped a week-long trip that also took him to Saudi Arabia and Britain.
IS was a focus of his private talks with his counterparts in all three stops.
Mr Obama said that in a meeting later with the leaders of Britain, Germany, France and Italy, he would ask those nations to step up their contribution to the air campaign and to the training of local forces.
He also said he would be seeking more economic aid to rebuild parts of Iraq the US-led coalition has recaptured from IS.
"Europe and Nato can still do more," he said. "We need to do everything in our power to stop them."
Mr Obama discussed his troop decision briefly during a broader speech on US-European relations and the importance to the world of continued European unity.
He urged Europe's leaders to pay attention to income inequality, which he said creates wedges among populations, and other issues including education for young people and equal pay for equal work for women.
"If we do not solve these problems, we start seeing those who would try to exploit these fears and frustrations and channel them in a destructive way," Mr Obama said, decrying an "us-versus-them" mentality that breeds animosity toward immigrants, Muslims and others.
Alluding to American presidential candidate Donald Trump, he said loud voices get attention when it comes to demonising minorities.
He added: "This is a defining moment and what happens on this continent has consequences for people around the globe.
"If a unified, peaceful, liberal, pluralistic, free-market Europe begins to doubt itself, begins to question the progress that's been made over the last several decades, then we can't expect the progress that is just now taking hold in many places around the world will continue."
The president's appeal for Europe to stick together came days after he made a forceful argument while in London against Britain exiting the European Union.
Mr Obama also said he wants good relations with Russia, but said the global community must keep up sanctions on the country until it fully implements its commitments under a Ukraine deal struck in Minsk.