Iran’s Islamic Revolutionary Guard Corps (IRGC) announced Saturday afternoon that the strategically vital Strait of Hormuz was closed to all shipping traffic, implicitly threatening commercial vessels attempting to transit between the Persian Gulf and the Gulf of Oman.

The UK Maritime Trade Operations Agency said it received multiple alerts from ships in the region reporting that the IRGC broadcast radio warnings declaring passage through the strait “not allowed.”

Sources in the trading industry told Reuters that “several tanker owners, oil majors and trading houses had suspended crude oil, fuel and liquefied natural gas shipments via the Strait of Hormuz.”

“Our ships will stay put for several days,” an executive at one trading desk told Reuters.

Satellite tracking data cited by Reuters showed oil tankers “piling up near major ports such as Fujairah in the United Arab Emirates” as operators sought to avoid the narrow waterway.

The Financial Times reported that insurers are preparing to cancel policies and sharply raise coverage costs for vessels operating in the Gulf and the Strait of Hormuz. Industry analysts told the FT that insurance premiums could rise by as much as 50% in the coming days, with companies already moving to cancel war-risk policies before markets reopen Monday. Coverage for ships sailing to Israel could increase by similar amounts.

“At least three ships turned away from the strait rather than pass through it, as shipowners assessed the risk of attack,” the FT reported.

However, Bloomberg News cited vessel-tracking data indicating mixed movement through the chokepoint, with seven ships exiting Hormuz and six entering after the IRGC warnings were issued.

Bloomberg also reported that Japan’s shipping giant Nippon Yusen KK instructed its fleet to avoid the strait, while Greece advised its merchant fleet to reassess transits through the area.