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Ukraine's President Volodymyr Zelenskyy on Saturday thanked the UK for the continuing training of members of the Ukrainian military on British soil.
British Prime Minister Boris Johnson spoke on the phone with the Ukrainian president, for an update on Kyiv's conflict with Russia, his Downing Street office said.
Offering a readout of the phone call, a No 10 spokesman said: "President Zelenskyy thanked the Prime Minister for the training of Ukrainian military personnel currently taking place in the UK."
It comes after Mr Johnson confirmed for the first time during his trade trip to India this week that Ukrainian forces were in the UK.
More than 20 soldiers arrived last week for training on the 120 armoured vehicles that are being supplied to the resistance against Moscow, the UK government has said.
The talks came as Ukrainian officials said Russian forces had started attacking a steel plant in the battered city of Mariupol in an apparent bid to eliminate the last stand by Kyiv's defenders.
An estimated 1,000 civilians remain sheltered in the Azovstal plant alongside the remaining 2,000 Ukrainian fighters.
President Vladimir Putin only two days ago ordered that Russian troops not be sent into the plant but instead that the factory be blocked off, in an apparent attempt to starve out the Ukrainians and force their surrender.
With the battle for Donbas raging in the east, the offensive marks renewed targeting of the south of the country.
During Mr Johnson and Mr Zelenskyy's telephone conversation, the Ukrainian leader provided an update on the situation in the Donbas and they both joined in condemning attacks by Russian forces against civilian targets, including in Mariupol and Odesa, as well as in Lviv in western Ukraine.
The prime minister said the UK was working to "develop a long-term security solution for Ukraine", as he also brought up international efforts to provide Ukraine with "further financial support" for the war, including conversations held at the G7 finance ministers' meeting on Wednesday.
A No 10 spokesman said Mr Johnson ran through the "defensive military aid" being supplied by the UK to help reinforce Ukraine's defenders, including drones and more anti-tank weapons, as well as the armoured vehicles that Kyiv's troops are being trained to use.
"The Prime Minister ended by reiterating the UK's unwavering support for the people of Ukraine and committed to continue working with international partners to provide the assistance necessary to help Ukraine defend itself," said the spokesman.
As well as being trained on the Mastiff, Wolfhound and Husky armoured vehicles, the Ukrainians are being shown how to use the Samaritan ambulance, and Sultan and Samson armoured reconnaissance vehicles.
In a press conference in Kyiv on Saturday, Mr Zelenskyy declared that he was "satisfied" with the level of military supporting coming from the UK.
He told reporters: "We want more than we're being given, but we're satisfied.
"We cannot refuse or reject anything during the war from the biggest military aid, which is coming from the United States and the United Kingdom.
"There are many other friends in Europe, but I'm talking about volumes of help and I'm grateful for it."
