"The missiles on civilian boats are a concern," said the official, who asked not to be named.

The person was confirming reports in The New York Times saying that Washington reacted to aerial photos from US intelligence agencies showing traditional boats carrying Iranian missiles in the Gulf, one of the world's most strategic waterways.

The fully-assembled missiles were loaded on the boats by Iran's Revolutionary Guards, which the US has designated a "foreign terrorist organization," the newspaper reported.

"What the military and the intelligence are concerned about is the intent," the US official told AFP.

The Trump administration has been ramping up pressure on Tehran for the past 10 days, reinforcing its military presence in the Gulf region to ward off what it warned were Iranian threats to US assets and allies there, without providing any evidence of the danger.

The Iranian ambassador to the United Nations, Majid Takht-Ravanchi, did not deny the shipment of the missiles in an interview with National Public Radio.

"We have to prepare ourselves," he said. "I am not in a position to talk about military preparedness in Iran. But what I can tell you is that all these allegations are part of fake allegations which are being directed against Iran to prepare something like a conflict or something like a war."

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THE NEW YORK TIMES COMPANY