The US claims to have multiple images of Iranian commercial vessels in the Persian Gulf that it believes are carrying missiles and other munitions, according to a US official with knowledge of the situation.
CNN has not reviewed the intelligence that has led to the US assessment. The US government has provided no proof the ships are carrying hidden missiles and other munitions.
The freighters have been modified, the official said, with large areas of their decks removed. The officials did not explain why these vessels would need to be altered to carry missiles.
Recent surveillance has shown some of those freighters moving in and out of Iranian ports in recent days, the official added.
At the same time, two US officials said that Iranian naval forces have significantly increased their presence around the Strait of Hormuz in the last two weeks.
The Pentagon declined to comment on the assessment.
CNN has learned of the images and the Iranian naval movements as the US has moved bombers and a carrier strike group into the region and fears of a conflict have been stoked by a sharp increase in tensions between Tehran and Washington.
Recent leaked US intelligence reports about the threat posed by Iran have sparked a debate within Washington about whether Iran is planning to attack US assets or whether it is acting defensively in an attempt to deter US action.
CNN has previously reported that small Iranian boats known as dhows have been transporting missiles around the Persian Gulf.
The two officials who spoke about the increased Iranian navy presence said it includes units from the Iranian Revolutionary Guard Corps maritime forces. The Trump administration designated the IRGC as a foreign terrorist entity in April.
The officials said there is no evidence the Iranian naval vessels are conducting a routine naval exercise in the area. They said that the movements do not reflect the typical Iranian pattern of transporting missiles outside the Persian Gulf to Iranian backed forces in Yemen.
There is also growing concern that the Iranians have significantly improved their ability to use drones, which could give them an enhanced intelligence about where US forces are located, the two sources said. They added that this increased capability could allow Iran to target US forces more precisely.
Critics are skeptical about US intelligence claims after the 2003 Iraq war, which was justified on the basis of now-discredited intelligence that Iraq possessed weapons of mass destruction.