Zelensky's arrival, confirmed by the UK government, comes after he secured fresh promises of military aid during weekend visits to France and Germany ahead of a widely expected counter-offensive against Russia.

"The UK is a leader when it comes to expanding our capabilities on the ground and in the air," Zelensky tweeted before meeting Sunak at the prime minister's country retreat of Chequers.

"This cooperation will continue today. I will meet my friend Rishi. We will conduct substantive negotiations face-to-face and in delegations," he said.

Sunak noted that the talks were taking place in the buildup to a Council of Europe leaders' meeting in Iceland -- which Zelensky will address by video -- and a G7 summit in Japan.

"This is a crucial moment in Ukraine's resistance to a terrible war of aggression they did not choose or provoke," the prime minister said in a statement.

"We must not let them down," he said.

"The frontlines of (President Vladimir) Putin's war of aggression may be in Ukraine but the fault lines stretch all over the world," he continued.

"It is in all our interest to ensure Ukraine succeeds and Putin's barbarism is not rewarded."

At Chequers, Sunak will confirm "the further UK provision of hundreds of air-defence missiles and further unmanned aerial systems including hundreds of new long-range attack drones with a range of over 200 km" (125 miles), the UK statement said.

"These will all be delivered over the coming months as Ukraine prepares to intensify its resistance to the ongoing Russian invasion."

The latest shipment comes after Britain last week become the first Western country to offer long-range cruise missiles to Ukraine, with its Storm Shadow rockets.

Britain is the second-largest provider of military assistance to Ukraine after the United States, and has added combat flight training to its programme of help, though not fighter jets.