Updated at 6:27 p.m. ET
President Trump has ordered some 1,500 troops to the Gulf region to serve a "mostly protective" purpose for American forces and interests.
Trump made the announcement to reporters on the White House lawn before boarding Marine One.
In a Pentagon briefing on Friday, Vice Adm. Michael Gilday, the director of the Joint Staff, would not say where the additional troops would be sent, other than that they would not be heading to Iraq and Syria.
Some of the forces have already arrived. A defense system designed to track and destroy incoming missiles, called a Patriot battery, is already in the region, staffed with about 600 troops.
The remaining 900 troops will operate intelligence surveillance radar and drones. A fighter squadron will also be deployed, though no specifics were provided.
The deployment of additional troops comes with the Pentagon's announcement that attacks on four oil tankers, carried out two weeks ago in the Persian Gulf, were the work of the Iran Revolutionary Guard Corps.
Gilday said the attackers used limpet mines, a type of mine that is attached to a vessel and then detonated.
He also blamed a rocket fired at the U.S. Embassy in Baghdad earlier this week on an Iranian proxy. Iran supports Shiite militias in Iraq, but the Pentagon did not identify which group had fired the rocket.
There were no reported injuries in either incident.
Acting Defense Secretary Patrick Shanahan told The Washington Post Thursday that the deployments were about deterrence, "not war."
Trump withdrew the United States from the 2015 Iran nuclear accord a year ago, and the rhetoric between Tehran and Washington has been increasingly belligerent. Iran recently announced that it would accelerate uranium enrichment if other countries don't help it mitigate economic sanctions imposed by the Trump administration.
Also on Friday, Secretary of State Mike Pompeo announced the U.S. is selling $8.1 billion in weapons and military supplies to Saudi Arabia, the United Arab Emirates and Jordan. In a written statement, Pompeo said the sales are meant to "deter Iranian aggression."
Pompeo said the equipment includes aircraft maintenance, munitions and other supplies. Letters provided to NPR show the sales include laser-guided bombs.
Members of Congress protested that the Trump administration is selling the arms without the usual congressional approval.
Sen. Bob Menendez, a Democrat from New Jersey who is the ranking member on the Senate Foreign Relations Committee, said the administration informed Congress that it was invoking an obscure provision that circumvents the usual procedure.
"In trying to explain this move, the Administration failed to even identify which legal mechanism it thinks it is using," Menendez said in a statement.
Arms sales to Saudi Arabia and the UAE have been criticized as supporting the war in Yemen, in which the Saudi-led coalition has repeatedly killed civilians and stands accused of indiscriminate airstrikes.
In his remarks Friday, Trump also discussed the possibility of pardoning military service members charged with war crimes. He told reporters that he hasn't "done anything yet" and was considering issuing pardons for some service members after their courts-martial.