The Pentagon has declined to say how many U.S. troops are actually on the ground in Iraq and Syria more than two years after the first deployments to fight the Islamic State.

The military only shares the number of full-time troops deployed, known as the "force management level" or FML.

That figure is currently about 3,825 in Iraq and 300 in Syria, but the number of troops on the ground, including temporary deployments, is much higher.

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There are an additional 800 to 900 U.S. troops and defense personnel temporarily deployed to Iraq, a figure that a defense official says "tends to run around."

It's unclear how many temporary troops are in Syria.

A Central Command spokesman acknowledged to The Hill that some troops that temporarily deploy aren't counted. In some cases, that's included senior officials on "personnel visits."

The issue has become a sticking point, with critics pressing the Pentagon for more transparency.

Some worry that officials are hiding the deepening U.S. involvement in the fight against the Islamic State in Iraq and Syria.

The pressure for the Pentagon to release the actual troop numbers comes as the administration faces questions from both parties about the strategy to fight ISIS and with no signs Congress is close to a deal on a war authorization.

The issue has been simmering for months. Defense officials have rejected repeated requests from reporters for the actual numbers.

"There's been a decision made not to release that number," Army Col. Steve Warren, a spokesman, told reporters on March 21. "The number that we release is our force management level... I don't have a reason for not releasing this number other than it's the orders that I'm under."

At a press conference days later, Joint Chiefs of Staff Chairman Gen. Joseph Dunford, Jr. said the military was "not reluctant" to give out actual troop numbers. But he added that the Pentagon had been consistent in not doing so for the last 15 years.

The Hill has also made repeated requests for more information on troop numbers, including a July 28 request for the number of temporary troops.

"We do not release those numbers due to the fluid nature of their presence; those numbers fluctuate on a daily basis," said Capt. Michael Meyer, a Centcom spokesman.

A request on Aug. 3 for a "ballpark estimate" also fell short.

On Wednesday, a defense official again said the actual number won't be made public, a decision from the office of the Defense Secretary and Centcom.

A spokesman for the Joint Chiefs said the number of troops deployed on a temporary basis can change widely day-to-day, and it would be too difficult to explain the numbers to the public.

Calculating the size of the anti-ISIS effort is a difficult affair, made complicated by the military's use of contractors and troops stationed throughout the Middle East.

An unofficial tally of all forces deployed to fight the Islamic State in Iraq and Syria indicates there could be as many as 8,000 personnel.

The current force management level is 3,825 in Iraq and 300 in Syria, There are 800 additional troops and personnel temporarily deployed to Iraq and another 100 U.S. troops at the Office of Security Cooperation in Baghdad, which technically falls under the State Department and is not counted.

And there are more troops on the way: The administration is deploying about 400 more troops to Iraq soon. The Pentagon room also has room to deploy an additional 422 troops at any time under caps set by President Obama.

Those deployments would bring the total number of troops in Iraq and Syria to 5,847 — well above the Pentagon's publicly released force management level.

But even that number doesn't include troops involved in the ISIS fight stationed elsewhere in the Middle East, outside of Iraq and Syria.

Centcom told The Hill on May 4 that there were 700 additional U.S. troops fighting ISIS in the region. But weeks later, Col. Warren publicly said the number was actually "several thousand others throughout the region and 1000s more back home."

That also doesn't include 1,605 American defense contractors in Iraq and an unknown number in Syria.

All in all, the total amount of troops and Defense Department personnel involved in the ISIS fight could be anywhere from to 8,252 to 10,152.

The use of temporary troops and contractors has also been a lightning rod for political controversy.

Some Republicans lawmakers say the military is using temporary deployments and contractors to sidestep "artificial troop caps" Obama imposed.

“If you are rotating people in every 30 days or whatever it is to keep below the troop caps then the people who are rotating in are not going to have time to get acclimated to the environment and may be at increased risk,” House Armed Services Chairman Mac Thornberry (R-Texas) said at a hearing in March.

Military officials say the troop caps make contractors necessary.

Army Vice Chief of Staff Gen. Daniel B. Allyn told reporters in June that the troop cap in Iraq forced the Army to break up units and rely more on contractors, which can be more costly.

The reliance on contractors has grown 15 percent so far this year, from 1,392 in January to 1,605 in July. The use of additional maintenance support grew 17 percent during that time.

The Pentagon is likely to face more pressure on force levels as ISIS looks to step up its attacks.

Some Republicans have said the administration needs to ramp up the levels quickly. But Democrats are wary of entrenching the U.S. in another protracted conflict.

For now, there is no sign officials will be more forthcoming.

The White House says it sets official troop levels but has no direct control over deployments or how the Pentagon explains those numbers.

"U.S. military forces are deployed to Iraq in accordance with the force management level set by the President and as required for temporary purposes as directed by the Secretary of Defense," said NSC spokesman Carl Woog.

"We refer you to the Department of Defense for specifics on their counting methodology."