Defense Secretary Ash Carter acknowledged Tuesday that the number of U.S. troops on the ground in Iraq is higher than the administration's authorized cap of 3,870.

"People who are temporarily assigned — and this has been true for here and in Afghanistan for some time — they, under the caps, are counted differently, as you well know," Carter told the House Armed Services Committee.

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"There are some people who are subject to the troop caps, and there are some people who rotate in for a short amount of time, that are not subject to the troop caps," he said.

Focus on the troop levels in Iraq came after a Marine was killed on Saturday in northern Iraq, revealing the quiet deployment of Marines to the country.

The Marines had deployed from an expeditionary unit that was already serving in the region, and they were not considered an additional deployment of troops because they were there on a temporary basis, defense officials said.

Troops who are serving in Iraq on a temporary basis, which includes many special operations forces, are not counted as part of the authorized troop level.

Sen. John McCain John Sidney McCainMcSally says current Senate should vote on Trump nominee Say what you will about the presidential candidates, as long as it isn't 'They're too old' The electoral reality that the media ignores MORE (R-Ariz.) said at a Christian Science Monitor breakfast in December that the real number was closer to 5,000, and a defense official said at the time the number was closer to 4,000. Defense officials have refused to give the complete number of U.S. troops on the ground in Iraq.

House Armed Services Committee Chairman Mac Thornberry (R-Texas) expressed concern on Tuesday over the administration's "artificial troop cap" was putting service members' lives at risk.

"The argument is, if you're rotating people in every 30 days or whatever it is to keep below the troop caps, then the people rotating in are not going to have time to get acclimated to the environment and may be at increased risk," he said.

"The other argument I've heard is when you have these artificial troop caps, you don't bring in the force protection that you would in other situations where you're not subject to those troop caps," he added.

Carter and Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford both said troops in Iraq have proper protection.

"Everybody that is in Iraq is properly trained for the mission, that included the Marines there," Carter said.

Dunford added that the deployment of more forces for protection or other critical capabilities has not been affected by the force cap of 3,870.

He added that the president has so far not turned down any such requests, and said he expects to have more as the U.S. ramps up its fight against the Islamic State in Iraq and Syria.

"To date, we haven't had any request that we've gone to the president with and this is now over the last several months for capabilities that has been denied," Dunford said.

"We're in the process right now of bringing forward recommendations for increased capability as a result of operations in Mosul, Raqqa and elsewhere, so we can maintain the momentum and accelerate the campaign."