Russian forces have seized a strategic Ukrainian port and besieged another as the two sides are set for another round of talks aimed at ending the war that has entered second week on Thursday.

Moscow's attempt to quickly take over the Ukrainian capital has apparently stalled, but the military has made significant gains in the south as part of efforts to sever the country's connection to Black and Azov seas, reports AP.

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

Elsewhere, the Russians pressed their offensive on multiple fronts, though a column of tanks and other vehicles outside the capital of Kyiv has made little progress in recent days. Heavy fighting continued Thursday on the outskirts of another strategic port city on the Azov Sea, Mariupol, plunging it into darkness, isolation and fear. Electricity and phone connections are largely down, and homes and shops are facing food and water shortages.

In just seven days of fighting, more than 2% of Ukraine's population has been forced out of the country, according to the tally of the UN refugee agency. The mass evacuation could be seen in Kharkiv, a city of about 1.4 million people and Ukraine's second-largest. Residents desperate to escape falling shells and bombs crowded the city's train station and pressed onto trains, not always knowing where they were headed.

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

As the toll of war mounted, a second round of talks between Ukrainian and Russian delegations was expected later Thursday in neighbouring Belarus — though the two sides appeared to have little common ground.

"We are ready to conduct talks, but we will continue the operation because we won't allow Ukraine to preserve a military infrastructure that threatens Russia," Russian Foreign Minister Sergey Lavrov said, repeating an accusation Moscow has repeatedly used to justify its invasion.

Lavrov said that the West has continuously armed Ukraine, trained its troops and built up bases there to turn Ukraine into a bulwark against Russia.

The US and its allies have insisted that Nato is a defensive alliance that doesn't pose a threat to Russia. And the West fears Russia's invasion is meant to overthrow Ukraine's government and install a friendly government — though Lavrov said Moscow would let the Ukrainians choose what government they should have.

Russian President Vladimir Putin earlier raised the spectre of nuclear war, putting his country's nuclear forces on high alert, but his foreign minister shrugged off questions of whether Russia could escalate the conflict with nuclear weapons, saying such talk comes from the West.

In Kherson, the Russians took over the regional administration headquarters, Hennady Lahuta, the governor of the region, said while adding that he and other officials were continuing to perform their duties and provide assistance to the population.

Kherson's mayor, Igor Kolykhaev, previously said that the national flag was still flying, but that there were no Ukrainian troops in the city. Britain's defence secretary said it was possible the Russians had taken over, though not yet verified.

Earlier Thursday, Ukrainian President Volodymyr Zelenskyy 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 Vitali Klitschko said explosions heard overnight in the Ukrainian capital were Russian missiles being shot down by air defence systems.

From Kherson, Russian troops appeared to roll toward Mykolaiv, another major Black Sea port and shipbuilding centre to the west along the coast. The regional governor, Vitaliy Kim, said that big convoys of Russian troops are advancing on the city but said that they will likely need to regroup before trying to take it over.

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

Moscow's isolation deepened when most of the world lined up against it at the United Nations to demand it withdraw from Ukraine. The prosecutor for the International Criminal Court opened an investigation into possible war crimes. And in a stunning reversal, the International Paralympic Committee banned Russian and Belarusian athletes from the Winter Paralympic Games.

While Moscow wreaked devastation on Ukrainian cities, global sanctions plunged Russia's economy deeper into crisis.

The ruble, which has tanked since the invasion, lost a further 15% against the dollar while the economy took another hit when two ratings agencies cut Russia's credit rating, saying the invasion and Western sanctions have hurt Moscow's ability to repay debts and raised risks for the economy and stability.

Russia reported its military casualties Wednesday for the first time in the war, saying nearly 500 of its troops have been killed and almost 1,600 wounded. Ukraine did not disclose its own military losses.

Ukraine's military general staff said in a Facebook post that Russia's forces had suffered some 9,000 casualties in the fighting. It did not clarify if that figure included both killed and wounded soldiers.

In a video address to the nation early Thursday, Zelenskyy 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 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."

Meanwhile, the senior US defence official said an immense Russian column of hundreds of tanks and other vehicles appeared to be stalled roughly 25 kilometres (16 miles) from Kyiv and had made no real progress in the last couple of 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.