Russia attacked one of Ukraine’s Danube river ports, ramping up the risks facing Kyiv’s last major grain export route and global food trade.

A drone attack overnight ruined a grain hangar at a Danube port, Ukraine’s southern operational military command said on Facebook. It didn’t specify which port had been hit, or give more details about the damage. 

It’s the latest in a series of moves by Russia to hinder Ukrainian exports, which have historically made an important contribution to global food supply. The collapse of the Black Sea grain deal last week, which allowed Ukraine to ship by sea, and Russia’s subsequent attacks on Odesa port spurred expectations that Ukraine will have to double down on alternative routes — the most obvious being the Danube river.

The volume of crops transported along the river reached 2.2 million tons in May, an increase of almost 900,000 tons from the end of last year. Those shipments even overtook the exports via the Black Sea corridor in May due to inspections slowing down the departure of freighters.

Wheat in Chicago rose as much as 6.9% on Monday, extending gains of more than 5% last week. Russia also unleashed a fresh missile barrage against Odesa at the weekend, the largest in a string of almost daily strikes on the Black Sea port city after Moscow pulled out of the grain deal. 

Ukraine has major ports of Reni and Izmail on the Danube at the border with Romania. Local traders including Nibulon and Kernel had been expanding capacity there in response to Russia’s sea blockade. Neither immediately responded to a request for comment.

“These ports are the biggest hope for Ukraine to export its grains and oilseeds.” Rabobank analyst Carlos Mera said. “We believe Ukraine could export up to 2.5 million tons of grains and oilseeds per month through these ports. That would be enough to export most of its exportable surplus. But at the moment it is unclear how much damage is done and if Russia will perform frequent attacks in the future.”

Russia is trying to “make it difficult” for Ukraine to export grain via the Danube, Ukrainian Agriculture Minister Mykola Solskyi told Bloomberg TV in an interview last week.