Donald Trump said on Sunday that U.S. forces would start blockading maritime traffic to Iranian ports, escalating tensions after negotiations failed to produce an agreement to end the conflict, putting a fragile two-week ceasefire at risk.

US Central Command said the blockade would take effect at 10 a.m. ET (1400 GMT) and would apply to all vessels entering or leaving Iranian ports across the Arabian Gulf and the Gulf of Oman. It added that freedom of navigation through Hormuz to non-Iranian destinations would not be impeded.

Shipping data showed several vessels adjusting routes in response. The Malta-flagged very large crude carrier Agios Fanourios I reversed course after attempting to enter the Gulf and is now anchored near the Gulf of Oman.

Other tankers, including the Pakistan-flagged Shalamar and Khairpur, continued operations, heading toward the United Arab Emirates and Kuwait respectively to load cargo.

The Liberia-flagged VLCC Mombasa B also transited the strait earlier on Sunday and is currently ballasting in the Gulf, according to vessel tracking data.Iran’s Islamic Revolutionary Guard Corps warned that any military vessels approaching the strait would be considered a violation of the ceasefire and would face a “decisive” response.

Despite rising tensions, shipping data showed that three fully laden supertankers passed through the strait on Saturday, appearing to be among the first vessels to exit the Gulf since the ceasefire agreement was reached last week.