(CNN) The Pentagon has rejected a request from the US Central Command for additional military resources in the Persian Gulf to beef up US deterrence against Tehran, according to two defense officials.

The request comes at a time when there are concerns the Iranian regime could potentially lash out militarily in response to the US reimposing sanctions on the country. It also comes as Defense Secretary James Mattis and Gen. Joseph Dunford, chairman of the Joint Chief of Staff, are engaged in high-level classified discussions with senior commanders around the world about how to allocate military resources for the next two years, officials say.

It's unknown publicly what exactly Gen. Joseph Votel, the four-star head of Central Command, asked for, but it's known that CENTCOM would like to see an aircraft carrier return to the Gulf in the coming months, as one has not been there since winter.

Dunford said Monday that the balancing of resource allocation is one of the toughest jobs in the military and it can disappointment some combat commanders.

"That is one of the more important tasks we face, is to prioritize and allocate resources in accordance with the strategy," he said. "And ... if you look at what the combatant commanders would identity as their requirements, our inventory falls short of meeting all of what the combatant commanders would want to have, so the art for us is to have enough of our forces forward to demonstrate commitment, assure our allies, to demonstrate the ability to respond -- which enhances deterrence -- and then have the inherent flexibility in the force to get the right amount of forces in the right place, at the right time in the event deterrence fails."

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