Russia's full-scale invasion of Ukraine has reached its fourth day and with it the sovereign state's second-largest city.
Fighting between the sides in Kharkiv comes as Ukraine have "engaged the remnants of Russian irregular forces" within its capital, Kyiv, according to UK defence.
Meanwhile, UK Foreign Secretary Liz Truss has warned there could be a conflict between Russia and NATO if Vladimir Putin is not stopped in Ukraine.
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Street fighting broke out in Ukraine's second-largest city on Sunday and Russian troops put increasing pressure on strategic ports in the country's south following a wave of attacks on airfields and fuel facilities elsewhere which appeared to mark a new phase of Russia's invasion.
The Ministry of Defence tweeted an intelligence update on the situation in Ukraine, saying: "Ukrainian forces have engaged the remnants of Russian irregular forces within the city of Kyiv for the second night in a row, fighting has been at a lower intensity than the previous evening.
"After encountering strong resistance in Chernihiv, Russian forces are bypassing the area in order to prioritise the encirclement and isolation of Kyiv.
"Intensive exchanges of rocket artillery overnight have been followed by heavy fighting between Russian and Ukrainian forces in Kharkiv.
"Russian forces are continuing to advance into Ukraine from multiple axis but are continuing to be met with stiff resistance from the Ukrainian armed forces."
Watch: On Saturday, Armed Forces Minister James Heappey discussed further munitions support being planned for Ukraine.
Ukraine's ambassador to the UK Vadym Prystaiko told BBC's Sunday Morning programme: "We're holding. It's been just four days. I believe that we'll hold as long as needed."
He said "we have friends, we have support", adding that he believes Russia has "miscalculated".
Mr Prystaiko said: "They came with 150,000 troops. It's totally not enough."
Foreign Secretary Liz Truss has warned an emboldened Russian leader could pose a risk to NATO territories if Russia is not stopped in Ukraine, which is not a member of the military alliance.
"This long-running conflict is about freedom and democracy in Europe," she said.
"If we don't stop Putin in Ukraine we are going to see others under threat – the Baltics, Poland, Moldova, and it could end up in a conflict with NATO.
"We do not want to go there. That is why it is so important we make the sacrifices now.
"Yes, there will be an economic cost here in Britain, there will be a cost in terms of access to oil and gas markets. I firmly believe that the British public understand the price we will pay if we don't stand up to Putin now."
Following its gains on the ground, Russia sent a delegation to Belarus for peace talks with Ukraine, according to the Kremlin.
However, Ukrainian President Volodymyr Zelensky suggested other locations, saying his country is unwilling to meet in Belarus because it served as a staging ground for the invasion.
He also said Russia should be thrown out of the United Nations Security Council.