Washington (CNN) US forces and air-defense missile batteries across the Middle East were placed on high alert overnight Monday to possibly shoot down Iranian drones as intelligence mounted about a threat of an imminent attack against US targets, two US officials told CNN.

On Tuesday night, Iran launched more than a dozen missiles at two Iraqi bases that host US troops in what appears to be an act of retaliation. The Islamic Revolutionary Guards Corps, an elite wing of the Iranian military known as the IRGC, said in a statement that the attacks were "hard revenge" for the death of Soleimani.

US intelligence had observed Iran moving military equipment, including drones and ballistic missiles, over the last several days. US officials said the movement may have been an Iranian effort to secure its weapons from a potential US strike, or put them in positions to launch their own attacks.

"There were indications that we needed to monitor the threats" even more closely than is already being done, one of the US officials said, referring to Monday night's state of heightened alert. The second official described it as "all Patriot batteries and forces in the area on high alert" against an "imminent attack threat."

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