The Pentagon said on Tuesday that two Iranian warships had linked up with a cargo ship that Iran says is carrying humanitarian aid to Yemen, adding it was monitoring the ships "every step of the way."

"We're not overly concerned at this point. It's a single ship that we've got very good accountability of," Pentagon spokesman Colonel Steve Warren told a news briefing.

Warren said the two warships "linked up" with the cargo ship when it passed an area where they had been conducting, according to Tehran, counter-piracy operations. Another Pentagon spokesman said this happened on Monday.