Residents of southern Ukraine, some who spent the night on rooftops, braced for a second day of swelling floodwaters on Wednesday as authorities warned that a Dnieper River dam breach would continue to unleash pent-up waters from a giant reservoir.

Officials said waters were expected to rise further following Tuesday’s dramatic rupture of the Kakhovka dam about 44 miles to the east of the city of Kherson, but some slowing was seen.

Ukraine accused Russian forces of blowing up the dam and adjoining hydroelectric power station, which sits in an area Moscow has controlled for more than a year. Russian officials blamed Ukrainian bombardment in the contested area, where the river separates the two sides.

Adding to the misery of the situation, Russian shelling regularly continues from across the nearby front line demarcated by the river.

Officials said about 22,000 people live in areas at risk of flooding in Russian-controlled areas on the eastern side of the river, while 16,000 live in the most critical zone in Ukrainian-held territory on the western side — areas like those evacuated on Tuesday.

Residents sloshed through knee-deep waters in inundated homes as videos posted on social media showed scenes including rescue workers carrying people to safety and what looked like the triangular roof of an entire building that had been uprooted drifting downstream. Footage taken from the air showed waters filling the streets of Russian-controlled Nova Kakhovska on the eastern side of the river.

The city’s Russian-appointed mayor, Vladimir Leontyev, said seven people were missing but early signs indicated that they could be alive. Officials in Russian-controlled parts of Kherson region said 900 Nova Kalhovka residents were evacuated, including 17 rescued from the tops of flooded buildings.

In Ukrainian-controlled areas on the western side, the head of the Kherson regional military administration, Oleksandr Prokudin, said in a video that water levels were expected to rise by another three feet over the next 20 hours.

“The intensity of floods is slightly decreasing; however, due to the significant destruction of the dam, the water will keep coming,” he said.

Britain’s Ministry of Defense, which has regularly issued updates about the war, said the Kakhovka reservoir was at “record high” water levels before the breach. While the dam wasn’t entirely washed away, the ministry warned that its structure “is likely to deteriorate further over the next few days, causing additional flooding.”

In another sign that what happens in Ukraine no longer stays in Ukraine, global prices for wheat and corn soared Tuesday soon after the explosion at the dam. 

Wheat prices gained 2.4 percent in early trading Tuesday on the Chicago Mercantile Exchange, to $6.39 a bushel. The cost of corn rose more than 1 percent, to $6.04 a bushel, and oats gained 0.73 percent, to $3.46 per unit. Prices were higher earlier in the day but faded.

The price rises renewed market fears about the fragility of Ukraine’s ability to ship food to Africa, the Middle East, and parts of Asia as it fights a war with Russia.

“Anytime this war shows signs of getting further escalated, there’s a lot of concern,” a senior research fellow at the International Food Policy Research Institute, Joseph Glauber, said. “Markets react to that.”

Ukraine and Russia are both major agricultural suppliers and the war’s disruption to their exports worsened a global food crisis tied to droughts and other factors. Breakthrough agreements brokered by the UN and Turkey last year got food moving again through the Black Sea, but it has faced setbacks.

Russia briefly pulled out of the deal last year, has threatened to leave again, is accused of slowing shipments from Ukraine and has only agreed to renew the deal for two months at a time. 

“People are going to be watching to see what happens with the agreement,” Mr. Glauber, former chief economist at the U.S. Department of Agriculture, said. “This reminds everyone that it’s not just pro forma, that this could be a very serious development if indeed the agreement is broken.”

There are huge agricultural fields in southern Ukraine where the dam burst, and while the collapse has endangered crops in the path of the floodwaters, Mr. Glauber says less wheat has been planted in that area because it’s near the fighting and a lot is growing elsewhere.

The collapse has threatened drinking water supplies, with officials also warning of a looming environmental disaster — pointing to oil escaping from the dam machinery and significant flooding.

The managing director of Black Sea agricultural markets research firm SovEcon, Andrey Sizov, said the dam collapse looked “like a big escalation with dire consequences and huge headline risk.”

A UN spokesman, Stephane Dujarric, said the flood caused by the dam breach was projected “to have severe and longer-term consequences on the humanitarian situation in the area,” such as by moving mines and explosive ordnance to new areas.