The U.S. on Thursday formally blamed Iran for attacks on two oil tankers near the Strait of Hormuz amid heightened tensions between the nations. 

Secretary of State Mike Pompeo made the accusation during brief remarks to reporters at the State Department, describing the attacks as part of "an unacceptable campaign of escalating tension by Iran."

The top diplomat said the U.S. assessment is based on intelligence, the weapons that were used, the level of expertise needed to carry out the attacks, as well as other recent attacks on ships in the region the U.S. has blamed on Iran. 

"No proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication," he said. "This is only the latest in a series of attacks instigated by the Islamic Republic of Iran and its surrogates against American and allied interests." 

He did not take questions.

The ships - one Marshal Islands-flagged and the other Panama-flagged -- were on their way to Taiwan and Singapore from Qatar and Saudi Arabia respectively when they were attacked near the Strait of Hormuz, which connects the Gulf of Oman with the Persian Gulf.

The strait is vital waterway with roughly one-third of the world's liquefied natural gas and a fifth of its oil consumption transiting through it each day.

The U.S. is expected to raise the attacks during a UN Security Council meeting later Thursday afternoon. 

They are the second in as many months near the Strait of Hormuz.

Late last month, the United Arab Emirates said four vessels, including two oil tankers, were hit by sabotage attacks near its territorial waters, without blaming anyone for the attacks.

The U.S. said the attacks were conducted using limpet mines, pointing the finger at Iran.

Saudi Arabia also said military drones carried out attacks on two oil pump stations on its East-West pipeline, which carries crude from the Abqaiq oil field to the Yanbu port on the country's Red Sea coast.

Riyadh and Washington blamed Iran-backed Houthi rebels in Yemen for the attacks.