The United States disclosed plans on Friday to station its first ground troops in Syria for the war against Islamic State, saying dozens of US soldiers would be sent as advisers to groups fighting the militants.

Washington announced the small special operations force shortly before 17 countries, the European Union and the United Nations called for a nationwide truce in Syria's civil war at talks in Vienna, attended for the first time since the conflict began in 2011 by President Bashar al-Assad's ally Iran.

The United States said it would deploy fewer than 50 troops to northern Syria beginning in the coming weeks in an open-ended mission. Officials said the forces were not meant for front-line combat.

The participants in Vienna, including the United States and Russia, said "substantial differences remain" though they agreed it was "imperative to accelerate all diplomatic efforts to end the war" and the ministers will reconvene within two weeks.

In a rare hint of diplomatic progress, Iran signalled it would back a six-month political transition period in Syria followed by elections to decide Assad's fate, although his foes rejected the proposal as a trick to keep Assad in power.

In addition to Assad's fate, on which delegates said no breakthrough had been expected, sticking points have long included the question of which rebel groups should be considered terrorists and who should be involved in the political process.

In Washington, US officials said the small special forces contingent in Syria would work with local "moderate rebel" groups to fight against Islamic State, also known as ISIS or ISIL, and that it should not be considered a combat mission.

"The president has been quite clear that there is no military solution to the problems that are plaguing Iraq and Syria. There is a diplomatic one," White House spokesman Josh Earnest said in Washington.

Earnest said the special forces' mission would be to "train, advise and assist" local groups, adding: "I think if we were envisioning a combat operation, we probably would be contemplating more than 50 troops on the ground."

In Alaska, US Defence Secretary Ash Carter told reporters, "Our role fundamentally and the strategy is to enable local forces, but does that put US forces in harm's way? It does, no question about it."

Carter did not rule out further special forces deployments to Syria if the initial one is successful.

Islamic State has seized swathes of eastern Syria and northern Iraq and proclaimed a caliphate to rule all Muslims.

The United States has conducted a bombing campaign against Islamic State targets in Syria since September 2014. But the deployment of even a small number of ground troops marks a significant change in Syria policy for President Barack Obama, who has been averse to committing troops to Middle East wars.

For example, Obama said in a nationally televised address in September 2013: "I will not put American boots on the ground in Syria."

The United States has acknowledged conducting special forces raids into Syria in the past including one by Delta Force commandos in May that killed a senior Islamic State leader, but it has not stationed troops there.

The decision is part of a package of other steps to beef up the fight against Islamic State, including sending more warplanes to the region and discussing with Iraq the establishment of a special forces task force there.

For Syria, it is part of what US officials call a two-pronged strategy of increasing aid to groups they describe as "moderate rebels" fighting against Islamic State, while also working on diplomacy to remove Assad from power.

KERRY TOUTS PEACE MOVES

In Vienna, US Secretary of State John Kerry said the timing of the announcement, as talks were held in the Austrian capital, was a coincidence and that peace moves must continue.

Russia's decision a month ago to join the conflict in Syria by bombing Assad's enemies has upended the strategy of the United States and its allies, who say Assad must go, as his presence makes it harder to fight Islamic State.

Russian Foreign Minister Sergei Lavrov said in Vienna that the decision to deploy special forces would make cooperation between US and Russian armed forces even more important.

Russia contends its bombing targets only Islamic State. But the overwhelming majority of its strikes have been against other groups fighting against Assad, including some that are supported by US allies.

For four years, Assad's closest ally Iran had been excluded from international peace conferences because it rejected a U.N.-backed proposal for a transition of power in Damascus. However, Tehran may be adjusting its stance in ways that could create more ground for compromise with Western countries.

"Iran does not insist on keeping Assad in power forever," Iranian Deputy Foreign Minister Amir Abdollahian, a member of Tehran's delegation at the Syria talks, was quoted by Iranian media as saying.

He later told Iranian state television that calls for a timetable for Assad's removal had been rejected at the talks and added: "The importance of the Syrian people deciding their country's fate was underlined."

Previous efforts toward a diplomatic solution to a civil war that has killed more than 250,000 people and driven more than 10 million people from their homes have collapsed over the insistence of the United States, European powers, Arab states and Turkey that Assad agree to leave power. – Reuters