"I have authorized approximately 1,000 additional troops for defensive purposes to address air, naval, and ground-based threats in the Middle East," Shanahan said in a statement.
The acting Pentagon chief noted that the deployment was requested by the US Central Command (CENTCOM) and advised by the chairman of the Joint Chiefs of Staff in consultation with the White House.
Shanahan pointed out that the decision resulted from the "hostile behavior" of the Iranian force, which "threatens US personnel and interests across the region."
The United States does not seek conflict with Iran, he said, adding "we will continue to monitor the situation diligently and make adjustments to force levels as necessary given intelligence reporting and credible threats."
The deployment came days after two oil tankers were attacked in the Gulf of Oman. The US administration has accused Iran of being responsible for the attacks, and the latter denied the accusation.
Meanwhile, US State Department spokesperson Morgan Ortagus announced on June 17 at a press briefing that Secretary of State Mike Pompeo will visit CENTCOM and US Special Operations Command in Florida on June 18.
"He will meet with General Kenneth McKenzie, commander of Central Command, and General Richard Clarke, commander of Special Operations Command, to discuss regional security concerns and ongoing operations," said Ortagus.
The Pentagon last month announced a decision to send to the Middle East 1,500 US troops along with Patriot air defense systems, drones, and fighter jets, a move Shanahan called a "prudent response to credible threats from Iran."
Washington and Tehran have been locked in a war of words in recent weeks amid escalating tensions following America's military buildup in the Middle East.
Iran has vowed to withstand the US "bullying policies."
Many analysts are concerned that unintended incidents and miscalculations between Washington and Tehran might ultimately trigger military conflicts.
