United Nations Secretary-General Antonio Guterres called on Friday for an independent investigation to establish the facts and who was responsible for attacks on two oil tankers this week in the Gulf of Oman.

The United States blamed Iran for the attacks on Thursday, a charge Tehran rejected. Amid the rising tensions, Guterres said he was available to mediate if the parties agreed, however, he added that “at the present moment we don’t see a mechanism of dialogue possible to be in place.”

U.N. political affairs chief Rosemary DiCarlo met with Iranian Foreign Minister Mohammad Javad Zarif on Friday on the sidelines of a Shanghai Cooperation Council meeting in Kyrgyzstan, U.N. spokesman Stephane Dujarric said. He added that Guterres and his staff had been in contact with many of the countries concerned to tell them of the need to avoid any escalation.

Tehran and Washington have both said they have no interest in starting a war, but this has done little to assuage concerns that the two arch-foes could stumble into a conflict. Guterres said the world cannot afford a major confrontation in the Gulf.

Guterres, who condemned the attacks on the tankers, was speaking on Friday alongside Arab League Secretary-General Ahmed Aboul Gheit after the pair met.

Gheit told reporters that he did not believe an Arab country was “trying to obstruct naval lanes or to shoot itself in the foot by acting the way we have seen in the Gulf of Oman or in the Hormuz Strait.”