The US military said vessels that have no need to move between ports in the Gulf of Oman and the Strait of Hormuz should stay away, as tensions ratchet up in the region“Due to the current geopolitical situation, vessels which do not need to transit the area are advised to remain well clear to avoid unnecessary delays to their voyage,” the US Naval Forces Central Command said in an advisory on Oct 28 issued via the Joint Maritime Information Center.

“Vessels transiting this area are advised to plan additional transit time to allow for increased traffic density and military blockade enforcement operations,” the notice said.

It remains unclear what prompted the advisory. The US military last publicly issued a similar notice in mid-July, when it began its blockade of Iranian shipping Back then, it said that the blockade would not impede neutral passage through Hormuz, though ships should expect to be questioned on their intentions.

Traffic through the chokepoint has recovered in recent months, with shuttle tankers making up a big part of the traffic carrying crude oil. These ships make multiple runs in and out of the Persian Gulf to the Gulf of Oman, where other tankers unwilling to risk the Strait await to receive cargoes.

Dozens of crude tankers willing to take the risk have helped lift oil flows from the Persian Gulf to near prewar levels, while the US blockade has pushed Iranian crude exports close to zero. At the same time, Iran has been ramping up its attacks on ships to a wartime high of almost 30 in October.

The latest US advisory said that neutral vessels with neutral cargo or are in ballast sailing between these neutral ports may be subject to questioning, visit, and search to confirm their intentions regarding the blockade.

The US Central Command and the US Naval Forces Central Command – the naval unit that is part of Centcom and oversees the naval blockade of Iran – did not immediately respond to an emailed request for comment.