US President Barack Obama delivers remarks at an International Association of Chiefs of Police (IACP) conference and expo in Chicago October 27, 2015. [Photo/Agencies]

US President Barack Obama plans to deploy dozens of special operations forces to northern Syria to advise opposition forces in their fight against Islamic State.

"These forces do not have a combat mission," White House press secretary Josh Earnest told reporters Friday.

Earnest described the decision to send fewer than 50 US troops to Syria as "an intensification of a strategy the president announced more than a year ago" to train, advise and assist local forces fighting ISIS militants.

"The fact is, our strategy inside of Syria hasn't changed," he said.

"It will not be their primary responsibility to lead the charge up the hill," he added of the special operations troops.

Earnest did not rule out sending more troops to Syria if they succeed in helping erase gains made by ISIS.

The planned deployment adds to an increasingly volatile and complex conflict in Syria, where Russia and Iran have increased up their military support for President Bashar al-Assad's fight against rebels in the four-and-a-half year civil war.

Russia said when it began air strikes last month that it would also target the Islamic State militant group, but its planes have hit other rebel groups opposed to Assad, including groups backed by Washington.