(Bloomberg) — Iranian forces were likely responsible for the attacks that occurred Thursday on two tankers in the Gulf of Oman, according to a report seen by Bloomberg from DNK, the insurer of one of the ships.

The Norwegian insurer has raised its assessment on the threat to tankers in the Persian Gulf, Strait of Hormuz and North Western Gulf of Oman to high following the incident, it said in the report. Anti-ship missiles fired from a naval asset were probably used in the attacks, it said.

The attacks on two tankers on Thursday have stoked regional tensions, with oil tanker owners reluctant to send vessels to the region. The U.S. has said it will consider providing naval escorts for commercial ships travelling through the Strait of Hormuz, the chokepoint at the southern end of the Persian Gulf. The U.S. has blamed Iran for the attacks on both vessels, while Tehran denies the accusations.

The Front Altair tanker was carrying a full cargo of naphtha when it experienced three explosions at two-minute intervals around the middle of its starboard side and above the waterline, according to DNK. The other ship, Kokuka Courageous, which was full of methanol, experienced two explosions on its starboard side near the engine room.

In a previous intelligence report issued in the wake of attacks on vessels moored at the U.A.E.’s port of Fujairah last month, DNK said two of the four targeted vessels were hit on the stern and two on the side near their engine rooms. The insurer said the Iranian Revolutionary Guard Corps was “highly likely” to be culpable, in a statement on May 14, two days after the attacks took place.

Details on the crew’s evacuation from the Front Altair tanker also emerged in the latest report. The crew was forced to board an Iranian vessel, having already been picked up by a nearby commercial vessel. Iran’s navy “demanded that the crew were transferred to them,” DNK said in the circular. The crew was then taken to the Iranian port of Jask.

DNK, formally known as the Norwegian Shipowners’ Mutual War Risks Insurance Association, declined to comment on the report. The explosion on the Front Altair “was caused by a hostile attack,” it said in a statement Friday, without assigning responsibility.

The Kokuka Courageous’ crew was picked up by USS Bainbridge, but has now returned to the damaged vessel in preparation for it to be towed, said Commander Joshua Frey, a spokesman for the Bahrain-based Fifth Fleet. The vessel will head to Khor Fakkan in the U.A.E. for repairs, according to the president of the ship’s operator.