Two oil tankers, one from Norway and one from Singapore, were attacked in the Gulf of Oman on Thursday morning, prompting the Fifth Fleet of the United States Navy to provide assistance to the vessels.

The Norwegian tanker was being charted by Taiwan’s state-owned CPC Corp. A CPC spokesman said that the firm suspects that the vessel was “hit by a torpedo.” BSM Management, a company from Singapore that owns the other tanker, spoke of a “security incident” that had damaged the vessel.

Last month, four oil tankers were attacked in the Gulf of  Oman. The United States blamed Iran for that incident, in which naval mines were used, but Tehran has denied responsibility. Tensions in the Middle East have increased in recent months, especially after the US increased sanctions on Iran.

Read more: Two tankers attacked in Gulf of Oman — reports