The United States escalated its fight against the Islamic State in Syria, pledging the first open deployment of military boots on the ground.
The move came as US, Russian and other diplomats began a new peace effort that America hopes will hasten the departure of Bashar Assad.
Up to 50 special operations troops will be sent to assist Kurdish and Arab forces in northern Syria, American officials said.
The move marks a significant departure for President Barack Obama, who for years has resisted putting ground forces in Syria.
However he has gradually intensified the US military response to what counter-terrorism officials worry is a growing Islamic State threat in Syria and Iraq.
The troop announcement came as diplomats in Vienna representing 17 countries and the EU agreed to launch a new peace attempt to gradually end Syria’s long civil war.
The declaration avoided any decision on when Assad might leave, and its is not clear how many rebel groups would agree to a plan that does not result in his immediate departure.
Any ceasefire agreement that may come as a result of the peace effort would not include the Islamic State, which controls large parts of northern Syria and has its capital there.
But the participation by Russia and Iran in the attempt could mark a new and promising phase in the diplomacy since those countries have staunchly backed Assad.
The White House has long said ousting Assad is essential to its ultimate goal of defeating IS as his brutal tactics against Sunni rebels have drawn Sunni radicals from all over the world into the militant group's ranks.
The Syrian civil war has killed more than 250,000 people and uprooted more than 11 million, sparking a refugee crisis throughout Europe.
Despite killing as many as 12,000 militants, the US bombing campaign has not significantly weakened the capacity of IS to hold territory, and the group’s ranks have been replenished by foreign fighters and others.
Military experts say ground troops are essential for the fight. A US programme to train Syrians was abandoned as a failure, and the new deployment essentially would replace it.
US defence secretary Ash Carter said this would probably not be the last significant adjustment to the anti-IS military campaign in Syria and Iraq.
“We are going to continue to innovate, to build up what works,” he said.
The developments came as missiles slammed into a crowded suburb of the Syrian capital Damascus, killing at least 40 people, activists said.
The Syrian Observatory for Human Rights and the Local Co-ordination Committees group said government forces fired more than 11 missiles at a market in the suburb of Douma.
In Washington, officials said the new US forces will work from headquarters locations and will not move to the front lines or be used to call in airstrikes.
However, the US has conducted special operations raids into Syria before now and will continue to do more unilateral raids.
It helped Iraqi forces rescue hostages last week, and Master Sergeant Joshua Wheeler was killed, the first US combat death in that country since 2011.
In Vienna, US secretary of state John Kerry said the countries at the talks - including fierce regional rivals Iran and Saudi Arabia – each pledged to support an independent and secular Syria, to maintain the country’s institutions, to protect the rights of all Syrians and to strive to defeat IS.