Russian forces have battled for control of a vital energy-producing city in Ukraine’s south and also gained ground in their bid to cut off the country from the sea, as Ukrainian leaders called on citizens to wage guerrilla war against the invaders.

The fighting at Enerhodar, a city on the Dnieper River that accounts for about one-quarter of the country’s power generation, came as the two sides met for another round of talks aimed at stopping the bloodshed that has set off an exodus of over one million refugees.

The mayor of Enerhodar, the site of the biggest nuclear plant in Europe, said Ukrainian forces were battling Russian troops on the city’s outskirts. Dmytro Orlov urged residents not to leave their homes.

Moscow’s advance on Ukraine’s capital in the north has apparently stalled over the past few days, with a huge armoured column outside Kyiv at a standstill. And stiffer than expected resistance from the outmanned, outgunned Ukrainians has staved off the swift victory that Russia may have expected.

A top Russian officer, Major General Andrei Sukhovetsky, commander of an airborne division, was killed in the fighting earlier this week, an officers’ organisation in Russia reported.

But the Russian forces have brought their superior firepower to bear in the past few days, launching missile and artillery attacks on civilian areas and making significant gains in the south as part of an effort to sever the country’s connection to the Black and Azov seas.

Cutting Ukraine’s access to the coastline would deal a crippling blow to the country’s economy and allow Russia to build a land corridor stretching from its border, across Crimea, which has been occupied by Russia since 2014, and all the way west to Romania.

The Russians announced the capture of Kherson, and local Ukrainian officials confirmed that forces have taken over local government headquarters in the vital Black Sea port of 280,000, making it the first major city to fall since the invasion began a week ago.

Heavy fighting continued on the outskirts of another strategic port, Mariupol, on the Azov Sea, plunging it into darkness, isolation and fear. Electricity and phone service were largely down, and homes and shops faced food and water shortages.

Without phone connections, medics did not know where to take the wounded.

A second round of talks between Ukrainian and Russian delegations began in neighbouring Belarus.

But the two sides appeared to have little common ground going into the meeting, and Russian President Vladimir Putin warned Ukraine that it must quickly accept the Kremlin’s demand for its “demilitarisation” and declare itself neutral, formally renouncing its bid to join Nato.

Mr Putin has long contended that Ukraine’s turn toward the West is a threat to Moscow, an argument he used to justify the invasion.

Mr Putin told French President Emmanuel Macron that he was determined to press on with his attack “until the end”, according to an official in the French president’s office.

Despite a profusion of evidence of civilian casualties and destruction of civilian infrastructure by the Russian military, some of it documented by The Associated Press, Mr Putin also called accusations that his military had attacked residential areas part of “an anti-Russian disinformation campaign” and insisted that Russia uses “only precision weapons to exclusively destroy military infrastructure”.

Ukrainian negotiator David Arakhamiya said Kyiv’s main demand in the talks would be the establishment of humanitarian corridors so civilians could reach safety. Ukraine also sought a cease-fire.

Ukrainians still in the country faced another grim day. In Kyiv, snow gave way to a cold, grey drizzle as long lines formed outside the few pharmacies and bakeries that remain open.

New shelling was reported in the northern city of Chernihiv, where emergency officials said at least 22 civilians had been killed in a Russian bombardment of a residential area. They warned that the number of casualties could grow as rescuers search the debris. The mayor said he was struggling to organise safe passage for civilians.

Families with children fled via muddy and snowy roads in the eastern region of Donetsk, while military strikes on the village of Yakovlivka near the eastern city of Kharkiv destroyed 30 homes, leaving three dead and seven injured, and rescuers pulled 10 people from the ruins, according to emergency authorities.

Ukrainian authorities called on the people to wage guerrilla warfare against Mr Putin’s forces by cutting down trees, erecting barricades in the cities and attacking enemy columns from the rear.

“Total resistance. … This is our Ukrainian trump card and this is what we can do best in the world,” Ukrainian presidential aide Oleksiy Arestovich said in a video message, recalling guerrilla actions in Nazi-occupied Ukraine during the Second World War.

In just seven days of fighting, more than 2% of Ukraine’s population has been forced out of the country, according to the tally the UN refugee agency released to The Associated Press.

The mass evacuation could be seen in Kharkiv, Ukraine’s second-largest city, with about 1.4 million people. Residents desperate to escape falling shells and bombs crowded the railroad station and pressed onto trains, not always knowing where they were headed.

At least 227 civilians have been killed and 525 wounded in that time, according to the office of the UN High Commissioner for Human Rights, though it acknowledged that is a vast undercount, and Ukraine earlier said more than 2,000 civilians have died. That figure could not be independently verified.

Meanwhile, a senior US defence official said the immense Russian column of hundreds of tanks and other vehicles appeared to be stalled roughly 16 miles from Kyiv and had made no real progress in the last few days.

The convoy, which earlier in the week had seemed poised to launch an assault on the capital, has been plagued with fuel and food shortages, the official said.

Ukrainian President Volodymyr Zelensky said that Russian land forces have stalled and Moscow is now unleashing air attacks, but that they are being parried by Ukrainian defence systems, including in Kherson.

“Kyiv withstood the night and another missile and bomb attack. Our air defences worked,” he said. “Kherson, Izyum — all the other cities that the occupiers hit from the air did not give up anything.”

Kyiv mayor and former boxer Vitali Klitschko said explosions heard overnight in the Ukrainian capital were Russian missiles being shot down by air defence systems.

In Kherson, the Russians took over the regional administration headquarters, said Hennady Lahuta, governor of the region. But he added that he and other officials continued to perform their duties.

From Kherson, Russian troops appeared to roll toward Mykolaiv, another major Black Sea port and shipbuilding centre to the west. The regional governor, Vitaliy Kim, said that big convoys of Russian troops were advancing on the city.

A group of Russian amphibious landing vessels also headed toward the port of Odesa, farther west, the Ukrainian military said.

Russia reported its military casualties on Wednesday for the first time in the war, saying nearly 500 of its troops have been killed and almost 1,600 wounded. Ukraine insisted Russia’s losses are many times higher but did not disclose its own military casualties.

In a video address to the nation early on Thursday, Mr Zelensky praised his country’s resistance.

“We are a people who in a week have destroyed the plans of the enemy,” he said. “They will have no peace here. They will have no food. They will have here not one quiet moment.”

He said the fighting is taking a toll on the morale of Russian soldiers, who “go into grocery stores and try to find something to eat”.

“These are not warriors of a superpower,” he said. “These are confused children who have been used.”

Around Ukraine, others crowded into train stations, carrying children wrapped in blankets and dragging wheeled suitcases into new lives as refugees.

Among the million-plus refugees who have fled Ukraine in recent days were some 200 orphans with severe physical and mental disabilities who arrived from Kyiv by train in Hungary on Wednesday.