The U.S. blamed Iran for attacks on two tankers in the Gulf of Oman on Thursday, saying the assaults were the latest in a series of hostile actions meant to disrupt the flow of oil.

“Taken as a whole, these unprovoked attacks present a clear threat to international peace and security, a blatant assault on the freedom of navigation, and an unacceptable campaign of escalating tension by Iran,” Secretary of State Mike Pompeo said, vowing the U.S. would defend itself and its partners.

The...