The Syrian Democratic Forces (SDF) announced on Tuesday the start of a "liberation campaign" in the northern Raqqa countryside.

The SDF is an alliance of Arab and Kurdish militias that have carved out broad swaths of territory in north and northeastern Syria. Its main fighting force is the Kurdish YPG militia, considered the best fighters against IS.

The offensive comes days after a rare visit to northern Syria by US Central Command Commander General Joseph L. Votel, who met with SDF forces to assess their strength. The visit prompted speculation the US-backed forces may be preparing for a large scale offensive.

Since early last year, Syrian Kurdish forces have taken significant chucks of territory from IS supported by US airstrikes. At the prodding of the United States, Syrian Kurdish forces formalized an alliance with some moderate Arab militias last year through the formation of SDF.

Since then, the US has provided the alliance with weapons and supplies. Last month, the Obama administration announced the US would deploy 250 special operations soldiers to join up with the SDF. They joined with 50 special forces sent last year.

The SDF said US special forces would be embedded with its units during the offensive.

The US aims to build up the Arab component of the SDF in preparation for an assault on Raqqa. The YPG is hesitant to drive too deep into non-Kurdish populated territory and want allied Arab forces to take the lead in any offensive on Raqqa.

The SDF is currently about 40 kilometers north of Raqqa. The SDF offensive comes as across the border in Iraq security forces and Shia militia make gains near the IS-stronghold of Fallujah. US-coalition airstrikes are supporting that operation.

The retaking of Fallujah could free up resources to advance on IS-held Mosul, Iraq's second largest city.