"There was a direct message relayed. That's as far as I'm going to go," US National Security Council spokesman John Kirby told reporters, declining to say how it was delivered.

The Pentagon says US and allied forces in Iraq and Syria have been attacked at least 16 times this month as tensions rise in the Middle East. It blamed "Iranian-backed militia groups."

Biden told a press conference on Wednesday that he had warned Khamenei of a response if the attacks continue.

"My warning to the Ayatollah was that if they continue to move against those troops, we will respond, and he should be prepared. It has nothing to do with Israel," he said.

Iran meanwhile warned Washington at the UN General Assembly over Israel's campaign of air strikes and artillery against Gaza.

"I say frankly to the American statesmen and military forces who are now managing the genocide in Palestine, that we do not welcome the expansion and scope of the war in the region," Iranian Foreign Minister Hossein Amir-Abdollahian said.

"But I warn if the genocide in Gaza continues, they will not be spared from this fire."

Later Thursday, Defence Secretary Lloyd Austin said US forces conducted precision strikes on two facilities in eastern Syria used by Iran's Islamic Revolutionary Guard Corps.

Austin also ordered Sunday the activation of air defense systems in the region and notified additional forces that they may be deployed soon.

The US has moved two aircraft carrier groups near Israel since the start of the conflict, in what it says is "a bid to deter Iran and its allies from broadening the conflict."