Testifying before Congress, John Rood, the under secretary of defense for policy, said the United States was "observing Iran's behavior with concern."

"We're continuing to look at that threat picture and have the ability to dynamically adjust our force posture," Rood told the Senate Armed Services Committee.

A US official told AFP on condition of anonymity that Defense Secretary Mark Esper was considering plans to move between 5,000 and 7,000 troops to the Middle East.

The official did not confirm where the troops would be sent, or in what timeframe, but said that the deployment would be due to frustrations with Iranian-linked groups' attacks on US assets.

Rood, under questioning, denied a report by The Wall Street Journal the United States was considering sending 14,000 more troops -- equivalent to the number sent over the past six months.

Esper also denied the 14,000 figure in a phone call with Senator Jim Inhofe, the chairman of the committee, Pentagon spokeswoman Alyssa Farah said.

US President Donald Trump later tweeted that: "The story today that we are sending 12,000 troops to Saudi Arabia is false or, to put it more accurately, Fake News!"

It was not immediately clear which report the president was referring to.

Tensions have risen sharply with Iran since Trump last year pulled out of a denuclearization pact and imposed sweeping sanctions, including trying to block all its oil exports.

In September, the United States said Iran was responsible for attacks on the major Abqaiq oil processing center in Saudi Arabia, a close US ally and Iran's regional rival.

Riyadh then asked Washington for reinforcements, receiving two fighter squadrons, additional missile defence batteries, and bringing the number of US troops stationed in the Kingdom to about 3,000.

The United States has also been alarmed by an uptick in attacks on bases in Iraq, where major demonstrations triggered by economic discontent have also targeted Iran's clerical regime and its overwhelming influence in its Shiite-majority neighbor.

"We're lucky no one has been killed. There is a spike in rocket attacks," another US official said.

"It's clearly not ISIS. Everything is going in the right direction and it's the right range," the official said, contrasting Iranian capabilities with those of the extremist Islamic State group.

Among the incidents, five rockets hit the Al-Asad Air Base on Tuesday, just four days after US Vice President Mike Pence visited US troops there.

Iran denied involvement in the September attack in Saudi Arabia, which was claimed by Yemen's Tehran-backed Huthi rebels.

The tensions come as Iran itself has faced major protests set off by a sharp hike in gas prices.