The U.S. blamed Iran for attacks on two oil tankers near the entrance to the Persian Gulf on Thursday as the incidents stoke fears that high-stakes diplomatic efforts won’t avert a military confrontation between the U.S. and Iran.

U.S. officials have determined Iran was responsible, Secretary of State Michael Pompeo told reporters in Washington, noting that Iran had previously threatened to curtail oil transport in the Strait of Hormuz.

Iranian officials have denied any involvement, with Foreign Minister Javad Zarif suggesting that Iran’s enemies may have been behind the attacks and reiterating calls for a regional dialogue.

“Suspicious doesn’t begin to describe what likely transpired,” Zarif wrote on Twitter earlier on Thursday.

The incidents, including an assault on a Japanese-operated vessel, were the second in a month to hit ships near the Strait of Hormuz chokepoint, through which about 40% of the world’s seaborne oil travels. They came as Japanese Prime Minister Shinzo Abe, a rare ally of both Donald Trump and Iranian leaders, visits Tehran in an effort to ease tensions. But Iran’s Supreme Leader, Ali Khamenei, told Abe on Thursday that his country would not repeat the “bitter experience” of talks with the U.S.

The prospects of a conflict have spiked since the Trump administration tightened its sanctions on Iranian oil exports in early May, following the president’s decision a year ago to withdraw from the 2015 Iran nuclear accord.

“The attacks put upward pressure on the probability of a US-Iran conflict in the Gulf (currently 30 per cent),” the Eurasia Group said in a note on Thursday. “The incidents appear aimed at demonstrating the vulnerability of Gulf shipping while damaging confidence in the U.S. ability to protect freedom of navigation.”

The U.S. last month expedited the deployment of a carrier battle group to the Middle East along with a Patriot missile battery and additional bombers as the Trump administration said it had evidence Iran was threatening attacks on American interests or allies in the region. At the same time, leaders on both sides repeatedly said they are seeking to avoid a war.