Wheat prices remain high as concern grows about Black Sea instability and disruption to the grain supply.

As Black Sea-bound vessels clustered in the waters near Istanbul, wheat prices remained elevated on Thursday, up 13 percent since Monday, when Russia pulled out of a wartime agreement that had been considered critical to stabilizing global food prices.

The termination of the deal, which had permitted Ukraine to safely export its grain through the Black Sea, could have significant long-term consequences for grain supplies, said Alexis Ellender, a global analyst at Kpler, a commodities analytics firm. Despite robust grain harvests from exporters including Brazil and Australia, prices could become volatile.

“By not having Ukraine there as a supplier, we’re increasing the vulnerability of the global grain market to these shocks,” Mr. Ellender said. “In the short term, supplies are good, but longer term, if we get any more supply shocks, we’re more vulnerable in terms of the global market.”

Another drought in Brazil, like in 2021, or a disruption to Australia’s barley and wheat crop caused by El Niño, could cause prices to soar, he said.

Russian threats to attack commercial vessels heading to Ukrainian ports have stalled traffic in the area. Marine tracking data shows that ships that had been en route to the Black Sea are sitting in ports in Istanbul as they wait to see if an agreement could be hammered out.

“They’re still deciding what they’re going to do,” he said. Some vessels could look to pick up shipments of grain from other parts of Europe.

At the moment, a quick resolution looks unlikely. Russia bombarded the port city of Odesa with missiles and drones on Tuesday and Wednesday, after an apparent Ukrainian drone strike on a Russian bridge linking the occupied Crimean Peninsula to mainland Russia.

The suspension of the deal between Russia and Ukraine also has implications for maritime insurers and shipowners, who will no longer have insurance coverage to travel to Ukrainian ports, said James Whitlam, a product director at Concirrus, a marine data and analytics platform. While the deal between Russia and Ukraine was in effect, ships were able to secure insurance coverage under a temporary agreement.

“Insurance markets are now scrambling around trying to understand what exposure they have,” Mr. Whitlam said.

Despite recent increases, grain prices are still lower than they were on the eve of Russia’s invasion of Ukraine in February 2022, partly because the end of the deal was expected, Mr. Ellender said. In addition, Ukrainian grain exports have recently been at reduced levels because of limited labor, with workers fighting the war, and limited fuel supplies and lost territory to Russia.

Ukraine has also increased exports by truck, train and river barge.

Ukraine is still likely to be able to export most of its wheat, corn, barley and sunflower seeds via alternative routes, said Rabobank, a Dutch bank, on Thursday. But this will put additional pressure on ports on the Danube River, which flows from the Black Forest in Germany to the Black Sea, and the cost of transport will become more expensive, and rail infrastructure will be at a higher risk of Russian attack, the note said.

“The higher transport cost means that Ukrainian farmers may, quite possibly, reduce planted area in the future,” the note said.

Ukraine is one of the leading exporters of grain and the leading global exporter of sunflower oil, and the deal had allowed Ukraine to restart the export of millions of tons of grain that dropped after the invasion.

Ukraine has exported 32.9 million metric tons of grain and other agricultural products to 45 countries since the initiative began, according to United Nations data. Under the agreement, ships had been permitted to pass by Russian naval vessels that had blockaded Ukraine’s ports in the aftermath of Russia’s full-scale invasion.

Soaring prices are expected to hit the poorest people in the world the hardest. Ukraine last year had supplied more than half of the World Food Program’s wheat grain sent to people in Afghanistan, Ethiopia, Kenya, Somalia, Sudan, and Yemen, according to the U.N.