A White House advisor said on Monday the US would send 250 special forces personnel to Syria in non-combat roles, five times the number of soldiers that are currently there, in the first major expansion of US ground troops since the war began.

President Barack Obama decided to send the troops to support local militias battling the Islamic State (IS) and speed up the process of retaking territory from the jihadists, Deputy National Security Advisor Ben Rhodes, told reporters in Hannover. The total number of US forces in Syria will now be 300.

"We've seen across parts of northern and eastern Syria progress as ISIL [an alternative acronym for IS] has been pushed out of some strongholds," Rhodes said. "We want to accelerate that progress and we believe the commitment of additional US special forces can play a critical role."

Obama, who is in Germany to discuss foreign policy with a number of European leaders, is expected to formally announce the deployment later on Monday.

Brett McGurk, Special Presidential Envoy to the American-led anti-IS coalition said the deployment — as well as that of additional Apache attack helicopters in neighboring Iraq, where IS also has a significant presence — would "further accelerate pressure" on the group's self-proclaimed capital of Raqqa in northern Syria and Iraq's second city of Mosul, which it also holds.

Obama has repeatedly ruled out sending ground troops to Syria, and the 50 US personnel already deployed there have also been described as serving in non-combat roles.

France 24 recently broadcast rare footage showing Western special forces alongside the US-backed Syrian Democratic Forces (SDF) directing airstrikes on IS from an A-10 Thunderbolt II overhead. The men were not wearing identifying insignia, but carried weaponry which are standard issue to American troops, including M4 carbines, a M249 light machine gun, an M2010 enhanced sniper rifle and a Milkor M32 multiple grenade launcher.

The Syrian conflict, which began in 2011, has now claimed well over 250,000 lives, forced four million people to seek refuge abroad and displaced some seven million more within the country. Dealing with IS, which has launched deadly terror attacks in Europe, is seen as an international priority, but within Syria the vast majority of civilian casualties have been caused by President Bashar al-Assad's forces.

IS has recently suffered a string of defeats both in Syria — where it lost control of Palmyra thanks to a Russian-backed offensive by pro-government forces — and in neighboring Iraq, where it was pushed out of Ramadi, the capital of Anbar province.