The United States says Iran is responsible for the tanker attacks in the Gulf of Oman -- but no group has claimed responsibility.

Secretary of State Mike Pompeo said, "This assessment is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication."

"Front Altair," owned by a Norwegian shipping company and "Kokuka Courageous" operated by a Tokyo-based shipping firm were attacked on Thursday in international waters near the strategically significant Strait of Hormuz.

The crews of both vessels were rescued, but one crew member was injured.

The UN Security Council held a closed-door emergency meeting Thursday afternoon at the request of the US.

The latest attack came as Japan's prime minister was on a visit to Iran to try and ease tensions between Tehran and Washington.

Pompeo said Iran insulted Japan by attacking a Japanese tanker.

NHK