US officials plan to station the first American boots on the ground in Syria in the war against Islamic State fighters, saying dozens of special forces troops would be sent as advisers to groups fighting against the jihadists.

The announcement of the small ground force came as diplomats from more than a dozen countries held talks over Syria, which for the first time in the more than four-year-old civil war were attended by President Bashar al-Assad's ally Iran.

In a rare hint of diplomatic progress, Tehran 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 the president in power.

The Vienna talks ended without a specific conclusion apart from an agreement to reconvene in some form next week, delegates said. In addition to Assad's fate, key 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 IS. Washington has targeted the group with air strikes for more than a year since fighters seized swathes of eastern Syria and northern Iraq and proclaimed a caliphate to rule over all Muslims.

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 the jihadists.

A senior US administration official said President Barack Obama had authorised sending fewer than 50 U.S. special forces troops to northern Syria to work with local groups. Washington has acknowledged conducting special forces raids into Syria in the past but has not stationed troops there.

Its main friends in northern Syria are Kurdish forces, who captured a swathe of territory from Islamic State along the border with Turkey over the past year with the aid of US air strikes. Washington has been cautious about publicly committing to helping the Syrian Kurds, who are mistrusted by US ally Turkey.

The measure would be 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 U.S. 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.

Russia, which started bombing a month ago, says it is targeting only Islamic State. But the overwhelming majority of its strikes have been against other groups fighting against Assad, including some that are supported by U.S. 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 appears to be adjusting its stance in ways that could create more ground for compromise with Western countries that are coming to accept that, as long as Assad is backed by Moscow, he cannot be driven from power by force.

"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.

A senior official familiar with the Iranian position said that could go as far as ending support for Assad after the transition period.

"Talks are all about compromises and Iran is ready to make a compromise by accepting Assad remaining for six months," the official told the Reuters news agency. "Of course, it will be up to the Syrian people to decide about the country's fate."

Assad's foes said such a proposal amounted to no new concession from Tehran, because a new election could keep Assad in power. His government held an election as recently as last year, which he easily won.

Nevertheless, a commitment from Iran to a defined time limit for a transition could be viewed as a significant undertaking, potentially forming a basis for future diplomacy at a time when Assad's position has been strengthened by Russia's decision to join the war on his side.

A senior US official and other delegates said a new round of Syria peace talks could be held as soon as next week.

All previous efforts to find a diplomatic solution to Syria's civil war have collapsed over the insistence of the United States, European powers, Arab states and Turkey that Assad agree to leave power.

Russia's participation in the conflict on Assad's behalf creates a new incentive for a diplomatic push to end a war that has killed more than 250,000 people and driven more than 10 million people from their homes.