WASHINGTON President Donald Trump may be on the brink of sparking a full-blown famine in Yemen all because of a subtle shift in messaging that risks effectively cutting off humanitarian relief to the war-torn nation.

For more than a year, former President Barack Obamas administration urged a coalition led by Saudi Arabia and the United Arab Emirates not to attack or seize the critical port of Hodeidah on Yemens west coast. The bulk of humanitarian supplies that enter Yemen flow through Hodeidah, and attacking the port, which is controlled by the Houthi rebels, would likely put it out of commission, the Obama administration warned.

But now the coalition, allied with the nominal Yemeni government of President Abdu Rabbu Mansour Hadi, has been redirecting humanitarian and commercial ships carrying food away from the Hodeidah port, relief workers told The Huffington Post. That redirection is itself a major blow to relief efforts. But it could also signal an attack is imminent.

For the past several weeks, the coalition has sent ships instead to the smaller port in Aden, about 250 miles away, said Jamie McGoldrick, a humanitarian coordinator for the United Nations. Humanitarian workers in Yemen have observed a drastic decrease in the number of ships reaching the port of Hodeidah, said Christophe Morard, a logistics officer at the U.N. World Food Program, which heads a group of nonprofits working together in Yemen.

The rerouting of ships coincides with coalition airstrikes over Hodeidah and a military offensive by the Saudi-led forces to retake Mokha , another port city about 100 miles to the south. That confluence of events has led aid workers to develop a contingency plan in case the coalition moves on from Mokha and closes Hodeidah in an attempt to retake it from the Houthis, Morard said.

Clearly they have some plans militarily for the port, McGoldrick said of the coalition. Its part and parcel of an attempt to try and weaponize the economy.

An effort by the Hadi-aligned coalition to retake Hodeidah would likely shut the port down for an extended period of time, former U.S. government officials and current aid workers say. Even a short-term halt of the flow of goods through Hodeidah would cut off life-saving food aid to Yemenis on the brink of starvation.

The Obama administration, which faced criticism from human rights groups for its military support to the Saudi-led coalition, took a hardline stance against the coalition attacking the Red Sea ports especially Hodeidah, four current and former administration officials told HuffPost. Obama specifically pressed the issue when Saudi Arabias King Salman visited the White House in 2015.

Even in a best-case scenario in which the coalition successfully retook the port from the Houthis, the battle would create front lines around Hodeidah, and theres ample reason to believe the Houthis would make repeated attempts to reclaim it. The passage of food through those lines would be as difficult as the passage through any other battles lines, one former senior administration official who worked on the region said, pointing to the southern port city of Aden as an example. When the coalition retook Aden in 2015, the port was inaccessible to humanitarian aid for four months, the BBC reported at the time.

Because the Houthis control major population centers in the western part of the country, its unclear whether the coalition would be able or willing to distribute humanitarian aid throughout Yemen if it succeeded in taking over Hodeidah. If we see a scenario where the bulk of the population is under the control of one side regardless of which side and the main channel for bringing in aid and commercial food is controlled by the other side, thats a recipe for disaster, said a second former administration official.