Boris Johnson has promised to provide additional military aid to Ukraine as President Volodymyr Zelensky told him what was needed to defend his country against Russian forces.
In a call on Saturday afternoon, the Prime Minister offered Mr Zelensky the UK’s “continued economic and humanitarian support”, a Downing Street spokesperson said.
Mr Johnson is said to have told the Ukrainian President that he is “more committed than ever to reinforcing Ukraine and ensuring Putin fails”.
Britain is one of the largest suppliers in Europe of arms to Ukraine, having already sent more than 5,000 anti-tank missiles, 1,360 anti-structure munitions, five air defence systems with more than 100 missiles, and 4.5 tonnes of plastic explosives, according to the Ministry of Defence.
Ukrainian forces have also been using Starstreak high-velocity and low-velocity anti-air missiles supplied by the UK.
Following the call between Mr Johnson and Mr Zelensky, a Downing Street spokesperson said: “The Prime Minister spoke to Ukrainian President Volodymyr Zelensky today, as part of their regular dialogue.
“President Zelensky updated on the fierce fighting in Eastern Ukraine and ongoing siege of Mariupol. He set out the equipment needed for the defence of Ukraine.
“The Prime Minister reiterated that he is more committed than ever to reinforcing Ukraine and ensuring Putin fails, noting how hard the Ukrainians are fighting for their freedom.
“He confirmed that the UK will continue to provide additional military aid to give the Ukrainians the equipment they needed to defend themselves.
“The leaders also discussed progress of the UN-led effort to evacuate Mariupol and concern for the injured there. The Prime Minister offered the UK’s continued economic and humanitarian support.
“The Prime Minister and President Zelensky agreed to remain in close contact on next steps, in coordination with international allies and partners.”
President Zelensky tweeted: “I keep in touch with @BorisJohnson. Spoke about the situation on the battlefield and in the blocked Mariupol.
“Discussed defensive support for Ukraine and the necessary diplomatic efforts to achieve peace.”
Meanwhile, the Foreign Office is investigating reports that a British national has been detained by Russia after a video emerged showing a man in camouflage clothes being questioned.
In the video, reportedly shown on Russian television, the man appears to give his name as Andrew Hill.
He speaks with an English accent, has his arm in a sling, a bandage around his head, and blood can be seen on his hand.
The video, which has not been verified, has been shared online.
The Foreign, Commonwealth and Development Office (FCDO) is investigating the reports and also supporting family members.
The FCDO condemns the exploitation of prisoners of war for political purposes and calls for anyone detained to be treated humanely in accordance with the requirements of international humanitarian law.
Two other British men, said to be working as humanitarian aid volunteers, are also believed to have been detained in Ukraine by Russian forces.
The Presidium Network, a non-profit group, said said Paul Urey and Dylan Healey were captured early on Monday morning at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine.
Mr Urey, who was born in 1977 and is from Manchester, and Mr Healey, born in 2000 and from Cambridgeshire, travelled to Ukraine of their own accord, the organisation said.
They were not working for the Presidium Network, which helps to get aid into Kyiv.
The organisation said the pair were driving to help a woman and two children to evacuate when they went missing.
Presidium Network said it is concerned Russian forces may think the two men are British spies.
The Foreign Office said it was urgently seeking more information following reports of British nationals being detained in Ukraine.
In an intelligence update on Saturday morning, the Ministry of Defence (MoD) in the UK said Russia faces “considerable challenges” and troops are likely to be suffering from “weakened morale”.
Posting on Twitter, the MoD said: “Russia hopes to rectify issues that have previously constrained its invasion by geographically concentrating combat power, shortening supply lines and simplifying command and control.
“Russia still faces considerable challenges. It has been forced to merge and redeploy depleted and disparate units from the failed advances in north-east Ukraine.
“Many of these units are likely suffering from weakened morale.
“Shortcomings in Russian tactical co-ordination remain. A lack of unit-level skills and inconsistent air support have left Russia unable to fully leverage its combat mass, despite localised improvements.”