U.S. President Barack Obama plans to deploy dozens of special operations forces to northern Syria to advise opposition forces in their fight against Islamic State, a major shift and a step he has long resisted to avoid getting dragged into another war in the Middle East.

The number of special operations troops in Syria would be fewer than 50, said a senior administration official, speaking ahead of an announcement on Friday by the administration. One U.S. official said the number was likely to be in the range of 20 to 30 but could not provide details.

The decision by Obama, deeply averse to committing troops to unpopular wars in the Middle East, would mark the first sustained U.S. troop presence in Syria and raise the risk of American casualties, although U.S. officials stressed the forces were not meant to engage in front-line combat.

The Obama administration is under pressure to ramp up America's effort against Islamic State, particularly after the fall of the Iraqi city of Ramadi to Islamic State in May and the failure of a U.S. military program to train and arm thousands of Syrian rebels.

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.

The new U.S. strategy to assist in the fight against Islamic State in Syria will be accompanied by a new special operations force in Erbil in northern Iraq, "intensified" cooperation with Iraqis in retaking Ramadi and expanded security assistance to Jordan and Lebanon, a senior congressional source said.

The U.S. special operations forces in Syria would be stationed in rebel-held territory, coordinate air drops to rebels and resupplying those forces as they move toward Raqqa, the declared capital of Islamic State, U.S. officials told Reuters.

They could also help coordinate air strikes from the ground, the officials said.

Regardless, the introduction of U.S. forces on the ground in Syria exposes the U.S. campaign to a new level of risk after more than a year of limiting the mission in Syria to a campaign of air strikes against Islamic State fighters.

To further counter Islamic State, Obama has also authorized deploying A-10s and F-15 aircraft to Incirlik air base in Turkey, the senior administration official said. The news came as U.S. Secretary of State John Kerry was meeting at Syrian peace talks in Vienna.

The talks include the foreign ministers of Russia and Iran, which support Assad, and nations such as Saudi Arabia and Turkey, which are adamantly opposed to his remaining in power after a civil war that has driven millions abroad as refugees and displaced millions more inside the country.