KHERSON: Ukrainians abandoned inundated homes on Wednesday as floods crested across the south after the destruction of a huge hydroelectric dam on front lines between Russian and Ukrainian forces, with their presidents trading blame for the disaster.

Residents slogged through flooded streets carrying children on their shoulders, dogs in their arms and belongings in plastic bags while rescuers used rubber boats to search areas where the waters reached above head height.

Ukraine said the deluge would leave hundreds of thousands of people without access to drinking water, swamp tens of thousands of hectares of agricultural land and turn at least 500,000 hectares deprived of irrigation into "deserts".

Ukrainian President Volodymyr Zelenskiy said in a video address that it was impossible to predict how many people would die in Russian-occupied areas due to the flooding, urging a "clear and rapid reaction from the world" to support victims.

"The situation in occupied parts of the Kherson region is absolutely catastrophic. The occupiers are simply abandoning people in frightful conditions. No help, without water, left on the roofs of houses in submerged communities," he said.

Visiting the city of Kherson downstream from the dam, Deputy Prime Minister Oleksandr Kubrakov said over 80 settlements had been affected by the disaster, and that the flooding had released chemicals and infectious bacteria into the water.

The Nova Kakhovka dam collapse on Tuesday happened as Ukraine prepares a major counteroffensive against Russia's invasion, likely the war's next major phase. Both sides traded blame for continued shelling across the populated flood zone and warned of drifting landmines unearthed by the flooding.

Kyiv said on Wednesday its troops in the east had advanced more than a kilometre around the ruined city of Bakhmut in eastern Ukraine, its most explicit claim of progress since Russia reported the start of the Ukrainian counteroffensive earlier this week. Russia said it had fought off the assault.

Oleksiy Danilov, secretary of Ukraine's national security council, said assaults under way were still localised, and the full-scale offensive had yet to begin.

"When we start (it), everyone will know about it, they will see it," he told Reuters.

Kyiv said several months ago the dam had been mined by Russian forces that captured it early in their 15-month-old invasion, and has suggested Moscow blew it up to try to prevent Ukrainian forces crossing the Dnipro in their counteroffensive.

Russian President Vladimir Putin accused Ukraine of destroying the dam at the suggestion of Western supporters, saying it was a "barbaric" war crime that escalated the conflict with Moscow. Putin described the incident as an "environmental and humanitarian catastrophe", according to a Kremlin read-out.

Neither side has presented public evidence demonstrating who was responsible. Some experts say the dam may have collapsed due to earlier war damage and poor Russian management.

'THEY HATE US'

Residents on the Ukrainian-controlled side of the flood zone in the south, a fertile, marshy region stretching to the Dnipro estuary on the Black Sea, blamed the bursting of the dam on Russian troops who held it on the eastern bank of the Dnipro.

"They hate us," said riverside villager Oleksandr Reva. "They want to destroy a Ukrainian nation and Ukraine itself. And they don't care by what means because nothing is sacred for them."

Russia imposed a state of emergency in the areas of Kherson province it controls, where many towns and villages lie in exposed lowlands below the dam.

In the town of Nova Kakhovka next to the dam, brown water submerged main streets largely empty of residents.

Over 30,000 cubic metres of water were gushing out of the dam's reservoir every second and the town was at risk of contamination from the torrent, Russia's TASS news agency quoted the Russian-installed mayor, Vladimir Leontyev, as saying.

Zelenskiy said he was "shocked" at what he called the lack of U.N. and Red Cross aid so far for victims of the disaster.

Shortly afterward, President Emmanuel Macron of France said on Twitter that "within the next few hours we will send aid to meet immediate needs".

The U.N.'s humanitarian affairs office said a team was in Kherson to coordinate relief efforts. Access to drinking water was a major concern and around 12,000 bottles of water and 10,000 purification tablets had been distributed so far.

Ukraine expects the floodwaters will stop rising by the end of Wednesday after reaching around five metres (16.5 feet) overnight, presidential deputy chief Oleksiy Kuleba said.

Two thousand people have been evacuated from the Ukrainian-controlled part of the flood zone and waters had reached their highest level in 17 settlements with a combined 16,000 people.