Acting Defense Secretary Patrick Chanahan said the ministry was considering sending additional troops to the Middle East at a time of heightened tension with Iran.
"What we are discussing is: is there anything we can do to strengthen the protection of forces in the Middle East? It may involve sending additional troops," Shanahan told reporters on Thursday.
But in statements to reporters outside the Defense Department, Shanahan rejected reports that specific numbers of troops were under investigation at this stage.
He noted that the Pentagon would report to President Donald Trump before deciding on the next step.
The US Defense Department is considering a request to send an additional 5,000 troops to the Middle East amid tensions with Iran, US media reported.
Shanahan stressed earlier that the military action in the Persian Gulf was aimed at deterring Iran rather than wage war against it.
Tensions have risen in recent months after the US administration imposed new sanctions on Iran, including its oil exports, and included the Iranian Revolutionary Guards on the list of terrorist organizations.
On the other hand, Iranian President Hassan Rowhani said on Thursday that his country "will not give in to American pressure and will not abandon its objectives even if it was bombed."