The U.S forces will include special operations teams as well as logistics and medical personnel, Cook said.
"They'll be engaging with forces on the ground, getting a better picture of the battle space, improving our intelligence assessment and our targeting assessment as well," Cook said.
"The original 50 are providing very, very helpful information," Cook told reporters Monday. "Those [initial] forces have improved our picture of the battlefield, made connections with local, capable forces and enhanced our targeting efforts in Syria. These new forces will expand those efforts and build on what has been working."
U.S. forces have reportedly set up a base camp at Rmeilan Air Base in the Syrian Kurdish region, providing a key supply line for U.S. operations in Syria, according to local reports.
The U.S. government considers many of the rebel groups in Syria to be extremists and not reliable partners for fighting the Islamic State. In some cases, American weapons ended up in the hands of groups linked to al Qaida or ISIS. Last year a train-and-equip effort was halted because very few Syrians could pass the strict vetting process.
"The idea is that they will not be engaged in direct combat, they will not be on the front lines. They will be providing support to those local forces that are taking the fight to ISIL," Cook said, referencing another acronym for the group.
Already, the president's critics in Congress have signaled their dissatisfaction with the plan, namely the small U.S. footprint in Syria compared to Iraq, where the American military presence has swelled to more than 4,000 troops.
On Monday Cook declined to say whether the U.S. will notify the Russians about the location of these additional forces.
"In the past we have identified, and we did identify a particular geographic area where we asked the Russians not to strike, and I am not going to discuss those conversations going forward, but we're going to take additional steps," Cook said.
"You can be sure to take every step we can to preserve the safety of our personnel and limit the risk they face. But I'm not going to speak to particular conversations, you know, we will have in the future or have had with the Russians at this point."
Andrew Tilghman is the executive editor for Military Times. He is a former Military Times Pentagon reporter and served as a Middle East correspondent for the Stars and Stripes. Before covering the military, he worked as a reporter for the Houston Chronicle in Texas, the Albany Times Union in New York and The Associated Press in Milwaukee.