(Reuters) - Saudi state oil firm Aramco has told Japanese refiner JXTG Nippon Oil & Energy about a possible change in shipment, raising concern about the kingdom’s ability to supply crude oil a week after attacks on its refineries, the Nikkei Asian Review reported.

Aramco did not specify a reason for the change in oil grade supplied to Japan’s biggest refiner from light to heavy and medium starting October, Nikkei said, citing JXTG officials.

JXTG officials suspect that Aramco is taking more time than expected to fix its desulfurization facility, which is necessary to produce light-grade crude used in the production of gasoline and light gas oil, the newspaper said.

At least three supertankers that loaded crude in Saudi Arabia this week for China and India had their crude grades switched from light to heavy oil while more buyers in Asia have been asked to delay shipments and switch grades in September and October, Reuters reported, citing sources and data from Refinitiv and Kpler.

The Sept. 14 attacks on the Abqaiq and Khurais plants, some of the kingdom’s biggest, caused raging fires and significant damage that halved the crude output of the world’s top oil exporter, by shutting down 5.7 million barrels per day of production.

Saudi Aramco has emerged from the attacks “stronger than ever”, chief executive officer Amin Nasser had told employees in a message on Saturday.

Yemen’s Houthi group claimed responsibility for the attacks but a U.S. official said they originated from southwestern Iran.

Aramco and JXTG did not immediately respond to requests seeking comment on the report.