US petroleum giant ExxonMobil said Sunday it was temporarily closing its enormous refinery in Baytown, Texas, after flooding caused by monster storm Harvey had led to "operational issues."

"We are in the process of a safe and systematic shutdown of operations," a company statement said.

ExxonMobil said it was "taking all precautions" to ensure workers' safety and minimize the impact of the closure on employees and the local community.

The group said all of its other Gulf Coast facilities were continuing to operate normally.

The Baytown complex is one of the biggest refining and petrochemical sites in the world.

Situated some 25 miles (40 kilometers) east of Houston, it produces an average 584,000 barrels of crude oil a day and employs 7,000 workers.

Harvey, the most powerful hurricane to strike the US mainland since 2005 and to hit Texas since 1961, roared onto the state's Gulf Coast late Friday as an immensely powerful category four hurricane.

It has since been downgraded to tropical storm status, but is continuing to drop record amounts of rain.

The area from Houston to Galveston was hit by some two feet (60 centimeters) of rain in 24 hours, and authorities have warned of catastrophic flooding of historic proportions.

Secretary of State Rex Tillerson, the former CEO of ExxonMobil, was asked about the storm during an appearance Sunday on Fox News Sunday.

He said his former industry had been through many major storms and was probably among the "most prepared for these types of events."

But, he added, "there will be challenges that are created by a storm of this magnitude."