Under the new plan, troops would protect a large swath of land controlled by Syrian Kurdish fighters.

The pullout occurred under the terms of a U.S.-brokered deal for a 120-hour pause in fighting expires Tuesday night.

The disappearance of the Adrian Darya 1, formerly known as the Grace 1, follows a pattern of Iranian oil tankers turning off their Automatic Identification System to try and mask where they deliver their cargo amid U.S. sanctions targeting Iran's energy industry.