As many as 250 U.S. Special Operations Forces and support staff are headed to Syria to increase support for local forces that are battling the self-proclaimed Islamic State, a defense official told The Daily Beast. And they probably won’t be the last.

The additional forces, which will increase the U.S presence in Syria fivefold, will begin arriving in the next two weeks and be a part of a “slow build-up,” the official said.

The U.S. troops are the latest to be deployed in a war that started with Obama administration officials promising there would be “no boots on the ground” but that now is seeing routine, incremental increases in troop deployments. This is the second deployment announcement in as many weeks, and officials at the Pentagon concede more will likely be deployed as the battles gear up to reclaim Mosul and Raqqa, ISIS’s de facto capitals in Iraq and Syria, respectively.

“This is the plan until this doesn’t work and then we put more in,” one defense official said, reflecting increasing skepticism that the U.S. will not be pulled further into the war.

Critics worry that the troop increases aren’t enough to defeat ISIS, but rather an attempt to contain the terror group with a minimum amount of U.S. military commitment.

“The mission is the same. The resources have been wholly inadequate. And we are still nowhere what we need to complete the mission,” said Christopher Harmer, a naval analyst at the Washington-based Institute for the Study of War. “To defeat ISIS in Raqqa and Mosul, there needs to be a ground force that has integrated close air support on request, along with integrated electronic and human intelligence enabling time sensitive strikes. None of the U.S. allies are remotely capable of doing us.”

The additional troops will also be put in charge of recruiting more Arab fighters, the official noted. Kurdish fighters have been among the most effective forces on the ground so far, but U.S. officials are reluctant to retake key Syrian cities without an Arab force in the lead. Kurdish fighters have been the most effective on the ground to date, but Kurdish officials have also said they don’t want to take the lead on an upcoming battle to retake Raqqa.

The additional troops on the ground also will help the U.S. to collect more intelligence, James Clapper, the Director of National Intelligence, told reporters at a breakfast in Washington on Monday. “Any time you get eyes and ears on the ground, that’s a good thing,” Clapper said.

But the administration’s top intelligence official would not predict whether adding more ground forces would change the trajectory of the war or hasten ISIS’s defeat.

The special forces headed to Syria are tasked with coordinating with Syrian Democratic Forces, a U.S.-backed group made up largely of Kurds, as they push west toward Raqqa. The Syrian forces have taken back several areas from ISIS in recent weeks, prompting the U.S. ramp up its own troop presence in order to take advantage of that momentum.

The U.S. troops are “not going to be leading the fight on the ground, but they will be essential in providing the training and assisting local forces that continue to drive ISIL back,” President Obama said Monday in Germany, using an alternative acronym for the terror group.

The troops are part of a two-part Pentagon plan that had been sitting at the White House for weeks and was only recently approved.

Last week, Secretary of Defense Ash Carter announced that the U.S. military would be sending an additional 217 troops to Iraq. Defense officials have signed off on sending a unit of troops deployed with HIMARs, or M142 High Mobility Artillery Rocket Systems, already assigned to the region, the defense official said, as part of that ramp-up of forces. The remaining troops will be made up of an additional group of Army advisers that will arrive from the United States. A small number of Navy personnel will be deployed, the official said.

The first known example of U.S. troops sending troops to the frontlines against ISIS was when 200 Marines arrived in the city of Makhmour, home to an Iraqi base is 70 miles south of Mosul. The Marines helped Iraqi forces make small advances toward Mosul on April 7, moving alongside their Iraqi counterparts. The U.S. didn’t announce the deployment of those troops until one of them, Staff Sgt. Louis Cardin, was killed March 19, days after arriving there.

—With additional reporting by Shane Harris.