Defence Secretary Mark Esper said that since May the United States has increased its 70,000-strong presence in the Middle East by 14,000 personnel
The Pentagon announced Friday it was bolstering US forces in Saudi Arabia after Riyadh asked for reinforcements following the September 14 drone-and-missile attack on Saudi oil plants which Washington blames on Iran.
Defence Secretary Mark Esper said that two fighter squadrons and additional missile defence batteries were being sent to Saudi Arabia, for a total of about 3,000 new troops from September this year.
The move comes as tensions jumped Friday after Tehran said that suspected missiles had struck an Iranian tanker in the Red Sea off the coast of Jeddah, Saudi Arabia.
Tehran did not blame arch-rival Riyadh for the attack, and US defence officials said they were still looking into it and had no immediate explanation.
Esper said he had spoken with Saudi Crown Prince Mohammad bin Salman Friday to discuss adding US firepower to the oil giant's defences against Iranian attacks.
"It is clear that the Iranians are responsible for the recent attacks on Saudi Arabian oil facilities," he said.
"Despite Iran's attempts to deny their involvement, the evidence recovered so far proves that Tehran is responsible for these attacks."
The September 14 attack knocked out two major processing facilities of state oil giant Aramco in Khurais and Abqaiq, roughly halving Saudi Arabia's oil production.
Washington condemned the attacks as a "act of war" but neither the Saudis nor the United States have undertaken overt retaliation.
But the incident added to tensions already soaring since early this year when Iran was accused of attaching mines to several tankers moored off Saudi Arabia and the United Arab Emirates, and then attacking or seizing others near the crucial Strait of Hormuz.
Esper said that since May the United States has increased its 70,000-strong presence in the Middle East by 14,000 personnel, most of those deployed to the Gulf region in response to Iran's actions.
"The US military has on alert additional army, navy, marine and air force units to quickly provide increased capability in the region if necessary," he said Friday.
He also urged US allies in Europe to follow America's lead with their own defensive assets "for regional stability."
The deployments authorised Friday include two additional fighter squadrons, and supporting personnel, along with additional Patriot and THAAD missile defense batteries.
The explosion on the Iranian tanker in the Red Sea Friday remained a mystery.
The National Iranian Tanker Company said the hull of the Sabiti was hit by two separate explosions off of Jeddah, saying they were "probably caused by missile strikes".
It initially blamed Saudi Arabia but then recanted the allegation, and no one claimed responsibility.
Oil prices surged about two percent on the news, which raised fresh concerns about a possible clash between the Iranians and Saudis.
"This clearly puts fuel on the Mideast fire," SEB commodities analyst Bjarne Schieldrop told AFP.
"After the attack on Saudi a few weeks ago... it's not a matter of if we get new comparable events -- but when, and how much."
But Ayham Kamel of the Eurasia Group expressed caution, saying that ongoing efforts to deescalate tensions would not be halted.
He said that if not perpetrated by the Saudis, the attack could have been carried out by Israel seeking to disrupt Iran's oil exports, the country's key source of income, or from a terrorist group.
But he said both Tehran and Riyadh have shown interest in calming the situation.
"For now, the attack will likely not derail efforts to ease tensions in the region and it does not materially impact the possibility for an escalation between Iran and Saudi Arabia," he said.