The US will send 1,000 extra troops to the Middle East to counter Iranian threats, it was announced on Monday night.
The news came after three rockets struck a military base in Iraq that is hosting US troops, the Iraqi military said.
And tension continued to escalate between the US and Iran as the Pentagon released photos it said were of Iranian troops near one of the oil tankers attacked in the Gulf of Oman.
Acting Defence Secretary Patrick Shanahan said that after consultations with the Pentagon and White House, "I have authorised approximately 1,000 additional troops for defensive purposes to address air, naval and ground-based threats in the Middle East”.
“The recent Iranian attacks validate the reliable and credible intelligence we have received on hostile behaviour by Iranian forces and their proxy groups that threaten US personnel and interests across the region.
“The United States does not seek conflict with Iran but to ensure the safety and welfare of our military personnel working throughout the region and to protect our national interests. We will continue to monitor the situation."
The three rockets hit Taji military base, 30 kilometres north of Baghdad, at about 10pm, the Iraqi military told Reuters.
Local reports said the rockets fired were Katyusha, but there was no immediate claim of responsibility for the attack and no casualties were reported.
An Iranian official called on the US to leave the region.
US officials are reviewing contingency plans and are preparing to send extra troops.
The US Defence Department also released close-up photos of Islamic Revolutionary Guard members removing the limpet mine from a Japanese tanker.
The photos are taken from a US Navy helicopter and offer a level of detail on what defence officials in Washington believe is a Guard boat approaching the Kokuka Courageous.
It also shows the damage from the mines on the tanker.
The US has accused the Iranian government of the attack and has been releasing digital evidence it believes backs up its claims.
US National Security Adviser John Bolton said Iranian threats to break the nuclear agreement and exceed the limit on enriching uranium by June 27 were “nuclear blackmail.”
Mr Bolton called for an increase in international pressure on Iran if it acted on its threat, and repeated that the US would not allow Iran to develop nuclear weapons.
Mr Bolton was also concerned over the security situation in the Gulf of Oman, he told The Washington Free Beacon.
“It's a very precarious situation,” he said.
After the attacks on six tankers in the UAE and the Gulf of Oman in the past five weeks, Mr Bolton said the US was “very concerned about the dangers of the Quds Force and Iranian intelligence operatives".
The Quds Force is the Guard's overseas operations unit.
The Trump administration blamed Iran for the attacks and has increased its military presence in the region to counter the threats.
Mr Bolton said that US President Donald Trump would be willing to reach a deal with Iran "assuming they give up nuclear weapons and stop the other malicious activity that they're engaged in".
Last month, the US also pledged to send additional troops to the Middle East, citing the growing threat from Iran.
“The deployment will include approximately 1,500 US military personnel and consist of a Patriot battalion to defend against missile threats, additional intelligence, surveillance, and reconnaissance aircraft, an engineer element to provide force protection improvements throughout the region and a fighter aircraft squadron to provide additional deterrence and depth to our aviation response options,” Mr Shanahan said in a statement on May 24.