The United States has concluded that the Islamic Republic of Iran is responsible for alleged attacks Thursday on one Norwegian and one Japanese tanker, Secretary of State Mike Pompeo announced at a press briefing.

Pompeo cited the “level of expertise needed to execute the operation” as a key factor informing the assessment. He added that “no [Iranian] proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication.”

There were conflicting reports sourced variously from representatives of the shipping companies and US government officials about the cause of the attack. The owner of the Japanese ship was quoted as saying that the vessel was hit twice with “some kind of shell.” A US official told Reuters that a mine was spotted near the same ship.

There were also reports that one shipping executive called it a “surface attack,” while another said that it was a suspected torpedo attack.

The Norwegian-MT Front Altair was set ablaze and burned for hours, a spectacle caught on camera and broadcast by Iranian state-owned news service Press TV. The crew was rescued from the flaming tanker by the Iranian navy, the outlet reported.

The price of crude oil jumped on the news, sending shares of energy companies up in New York trading on Thursday. Brent crude and West Texas Intermediate both jumped as much as more than 4% before settling up by about half that amount.

In his press briefing, Pompeo responded to Iranian Foreign Minister Mohammad Zarif’s Tweet that “suspicious doesn’t begin to describe what likely transpired this morning.”

“Foreign Minister Zarif may think this is funny, but no one else in the world does. Iran is lashing out because the regime wants our successful maximum pressure campaign lifted,” Pompeo said.

Despite the escalation in tensions, Pompeo signaled that the US will continue to limit its policy to economic pressure, which leaves few options for additional measures.

“Our policy remains an economic and diplomatic effort to bring Iran back to the negotiating table at the right time, to encourage a comprehensive deal that addresses the broad range of threats,” he said.