The shipping industry was placed on high alert on Sunday with warnings that Tehran could retaliate against commercial vessels following after U.S. airstrikes against Iran’s nuclear facilities.

Greece cautioned its ship owners to think again if considering entering the Persian Gulf in the wake of U.S. airstrikes. Vessels planning to sail through the Strait of Hormuz, the waterway that sits at the mouth of the region, should “reassess passage” until the situation normalizes, according to a circular, seen by Bloomberg that its shipping ministry sent to vessel owners. It advised waiting in nearby safe ports.

Naval forces in the area warned that ships, especially U.S.-linked ones, could be at heightened risk. The actions of the maritime industry — and its risk tolerance — will be a critical detail in the wake of the strikes because of Iran’s proximity to the strait of Hormuz, a conduit for a fifth of the world’s oil and an unavoidable sea route into the Persian Gulf.

Athens’ warning is the latest sign of pressure on shipping markets as attacks on Iran escalate. Tanker earnings already soared by almost 90% since Israel first started conducting airstrikes on June 13. As one of the world’s largest shipowning nations, Greece and its advice to vessel owners would have a major impact on commodity transportation markets, especially oil.

There’s every chance the advice will be ignored because the Persian Gulf is too important a region for shipowners to avoid and rates can always rise to compensate for the risk of sailing in the region. Shipowners that do decide to transit Hormuz should adopt the highest security level available and maintain the maximum possible distance from Iranian waters, Greece’s ministry added.

In Sunday’s notice, the Greek ministry cited concern around a possible closure of Hormuz as a reason behind its message.

Officials at three Greek tanker companies said they were still assessing the situation. One did indicate he might still allow his tankers to enter the region, while another said their ships would likely stay away.

Calls and emails to the ministry weren’t immediately responded to outside usual working hours.

Bigger risk

Naval groups are also warning of greater risk.

On Sunday, the Joint Maritime Information Center, a liaison between navies and merchant shipping in the region, said that the U.S. airstrikes mean U.S.-linked ships sailing through the Red Sea and Gulf of Aden face a high risk of attack.

Yemen’s Houthi rebel group issued fresh threats against U.S. commercial and naval ships earlier in the day. There had been a ceasefire between the U.S. and the Houthis in early May, geared toward limiting the group’s attacks on the U.S. navy. U.S.-linked ships should consider rerouting, the JMIC said in its update.

Separately, the European Union’s naval force in the region raised its threat assessment for U.S.-linked vessels as a result of the strikes. It now sees a severe threat to ships linked to the U.S. and Israel and a low risk for all other ships.

“This does not exclude the possibility of all merchant vessels being targeted in the future,” it said in an update published by France’s MICA Center, which helps coordinate global maritime security.

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(With assistance from Paul Tugwell.)

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