Washington (CNN) Intelligence showing that Iran is likely moving short-range ballistic missiles aboard boats in the Persian Gulf was one of the critical reasons the US decided to move an aircraft carrier strike group and B-52 bombers into the region, according to several US officials with direct knowledge of the situation.

The concerns over the movement of the missiles was one of multiple threads of intelligence from various sources that led the US to believe Iran had a capability and intention to launch strikes against US targets.

On Tuesday US Central Command spokesperson Capt. Bill Urban said they had seen "indications that Iranian and Iranian proxy forces were making preparations to possibly attack US forces in the region."

It's not clear if the Iran could launch the missiles from the boats or if they are transporting them to be used by Iranian forces on land.

The Pentagon is considering sending additional firepower to the region including anti-missile defense systems, the officials said.

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