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President Trump has commissioned about 1,500 troops to the Gulf region to serve a "mostly protective" purpose for US forces and interests.
Trump announced the announcement to journalists on the White House lawn before the Marine One table.
In a Pentagon briefing on Friday, director of the joint staff vice president Michael Gilday would not say where the extra troops would be sent, except that they would not approach 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, manned by about 600 soldiers.
The remaining 900 soldiers operate intelligence radar and drones. A fighter squadron will also be deployed even though no specifications were given.
The spread of additional troops follows with the Pentagon's announcement to attack four oil tankers carried out two weeks ago in the Persian Gulf, Iran's work was the Revolutionary Guard Corps.
Gilday said the attackers used limpet mines, a type of mine attached to a vessel and then detonated.
He also accused a rocket fired at the US Embassy in Baghdad earlier this week at an Iranian proxy. Iran supports Shia militias in Iraq, but the Pentagon did not identify which group had fired the rocket.
There were no reported injuries in both events.
Acting Defense Minister Patrick Shanahan told The Washington Post on Thursday that the implementations were about deterrence, "not war".
Trump withdrew the United States from the 2015 core convention, and the rhetoric between Tehran and Washington was increasingly warring. Iran recently announced that it would speed up the uranium enrichment if other countries did not help alleviate economic sanctions imposed by the Trump administration.
Also on Friday, congressmen said that the Trump administration is selling more weapons to Saudi Arabia and the United Arab Emirates without the usual congressional approval.
Sen. Bob Menendez, a New Jersey democrat who is a member of the Senate for the Foreign Relations Committee, said the administration advised Congress that it invoked an unclear provision to avoid congressional approval being usually required.
"To try to explain this step, the administration itself failed to identify which legal mechanism it believes it uses," Menendez said in a statement.
Arms sales to both countries have been criticized as supporting the war in Yemen, where the Saudi-led coalition has repeatedly killed civilians and is accused of arbitrary air strikes.
In his remarks on Friday, Trump also discussed the possibility of forgiveness of military service members charged with war crimes. He told reporters he hasn't "done anything yet" and was considering publishing congratulations to some service members after their courts.