Pentagon spokesman John Kirby said Tuesday the USS Nimitz carrier strike group had sailed from the US military's Central Command in the Middle East to the Indo-Pacific Command region.

Kirby did not confirm reports the Nimitz was headed back to the United States after some nine months at sea.

But he indicated that, after the Trump administration ramped up the US military presence in the Gulf, the Biden administration did not see keeping the carrier there as necessary for US security needs.

Kirby declined to discuss the current Pentagon assessment of an Iranian military threat to US bases or Gulf allies.

However, he said: "We don't make decisions like this lightly."

Defense Secretary Lloyd Austin "believes that we have a robust presence in the Middle East to respond" to any threat, Kirby added.

"The secretary was mindful of the larger geostrategic picture when he approved the movement of the carrier strike group from the Central Command area of responsibility to the Indo PACOM area of responsibility," he said.

Kirby would not say if the Nimitz would be replaced in the region in the near future, noting that the US Navy has a limited number of aircraft carriers.

"We're constantly watching the threat. We're constantly trying to meet that threat with proper capabilities," he said.