Live updates: follow the latest news on Russia-Ukraine
British Prime Minister Boris Johnson met President Volodymyr Zelenskyy in Kyiv on Saturday.
The UK said it would provide 120 armoured vehicles, more anti-ship missiles and guarantee an additional $500 million in World Bank lending to Ukraine.
“Boris Johnson's visit in Kyiv began just now with a tête-à-tête meeting with President Zelenskyy,” presidential aide Andriy Sybiha said on Facebook, posting a photo of the leaders.
Appearing alongside Mr Zelenskyy in a recorded broadcast clip, Mr Johnson said the West would continue to "ratchet up" sanctions on Moscow as he praised the courage of the Ukrainian resistance.
"I think that the Ukrainians have shown the courage of a lion, and you Volodymyr have given the roar of that lion," he said.
"The UK and others (will) supply the equipment, the technology, the know-how, the intelligence, so that Ukraine will never be invaded again.
"So Ukraine is so fortified and protected - that Ukraine can never be bullied again. Never be blackmailed again. Never be threatened in the same way again."
The armoured vehicles and anti-ship missiles are in addition to £100m ($130m) worth of military equipment announced on Friday, Downing Street said.
Britain would also guarantee an additional $500m in World Bank lending to Ukraine, taking its total loan guarantee to $1 billion, and would liberalise tariffs on most imports from Ukraine.
A video released by Mr Zelensky's office showed him and Mr Johnson walking through largely empty city streets to Kyiv's historic Maidan Square, as snipers kept watch.
The two men greeted passersby, and one visibly emotional man called out to Mr Johnson, "We need you."
A picture of the leaders' meeting was posted on social media by the Ukrainian embassy in London.
It took place the day after EU leaders were in Kyiv and Bucha, the town where Ukraine found what it said was evidence of a massacre of civilians after Russia troops withdrew.
European Commission President Ursula von der Leyen said on Saturday that Russian forces appeared to have committed war crimes by taking aim at civilians in Ukraine.
“My instinct says: If this is not a war crime, what is a war crime? But I am a medical doctor by training, and lawyers have to investigate carefully,” Ms von der Leyen said.
Russia is focusing its offensive, including sea-launched cruise missiles, on the eastern Donbas region, the British Ministry of Defence said in its daily briefing.
It said it expected air attacks would increase in the south and east as Russia seeks to establish a land bridge between Crimea, which Moscow annexed in 2014, and the Donbas.
Following Russia's withdrawal from around Kyiv, President Vladimir Putin has now appointed a commander who led Russian troops in Syria in 2015 to command the Donbas region offensive.
Gen Alexandr Dvornikov, 60, who was decorated by Moscow for his Syrian campaign, will be under pressure to deliver results before Russia’s Second World War celebration of Victory Day on May 9.
Analysts said haste to achieve results in Ukraine is leading Moscow’s commanders to repeat tactical errors, leaving their armoured vehicles vulnerable to attack by Ukrainians armed with anti-tank weapons.
Before the invasion, the VDV paratroopers — known as the “blue berets” — were feared as a well-honed force, equivalent to western counterparts such as Britain’s Parachute Regiment and US Airborne.
But paratroopers were sent into the fight underprepared and without the necessary armour to face the Ukrainian military, a senior western official said.
