OPEC Secretary General Haitham al-Ghais pushed back on the International Energy Agency's prediction that there will be a supply glut in oil markets following the end of the war between the U.S. and Iran.

The war is ​estimated to have blocked more than 14 million barrels per day of Middle East oil output according to the IEA.

"If the deal holds, exports and production from the Gulf should see a gradual recovery — not least because Iranian oil exports can fully ⁠resume once the U.S. blockade is lifted," the agency, which advises industrialized countries, said.

The oil market will then fall into a significant supply overhang ​next year, the IEA said in its first look at 2027, as oil supply is set to surge by 8 million bpd while demand ⁠rises by just 2 million bpd.

But al-Ghais told CNBC the IEA is way off.

"Sometimes it's best not to make such assumptions when they are not really based on facts and figures," al-Ghais said. "What does the IEA see that OPEC and the rest don't see?"

"[We focus] on fundamentals and not putting many ifs and buts in our forecasts, but rather focusing on actual numbers," al-Ghais added.

"We're not about making a fancy headline that will be catchy. Sometimes it's best not to make such assumptions ... when they are not really based on actual facts and figures," al-Ghais continued. "Ultimately, these headlines only create more volatility."

Flows through the strait were already rising by early June because of a pick-up in ship-to-ship transfers in the Gulf of Oman, the IEA said, helping to boost total Middle East flows to around 12 million bpd in early June from a May low of 9.6 million bpd.

However, political and operational constraints, including prolonged ‌demining and unresolved transit arrangements, leave downside risks to the Middle East recovery outlook, the IEA said.

Overall, the IEA forecasts oil supply to ​fall by 3.9 million bpd ‌in 2026, as production losses in the Middle East outpace rising output from the Americas.

al-Ghais said it is premature to judge the impact of the ending of the U.S.-Iran war.

"I think what the last four months have really proven is just how critical that waterway is not just for OPEC producers, but for Middle Eastern producers and global energy markets," he told CNBC.

Reuters contributed to this report.