Iran seized another oil tanker on May 3, making it the second seizure in under one week. The seizure is adding to concerns that Iran is threatening maritime traffic in the open seas of a pivotal strait where nearly one-fifth of all crude oil passes through.

Surveillance video from the U.S. Navy shows about a dozen of Iran’s paramilitary Revolutionary Guard vessels swarming tanker “Niovi” in the strategic Strait of Hormuz. The tanker was flying a Panama flag.

The aerial drone video shows Guard ships forcing the oil tanker to reverse course and head toward Iranian waters off the coast of Bandar Abbas, Iran.