Dressed in his trademark military fatigues, Zelensky gave a bear hug to Sunak when they met at chequers, the prime minister's country retreat northwest of London.

The trip came after the Ukrainian leader 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 his talks with Sunak.

"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 build-up 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 (Russian President Vladimir) Putin's war of aggression may be in Ukraine but the fault lines stretch all over the world.

"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 kilometres" (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 became the first Western country to offer long-range cruise missiles to Ukraine, with its Storm Shadow rockets.

Britain has added combat flight training to its programme of support, though not fighter jets.

On only his second trip overseas since the war began, Zelensky in February gave a much-acclaimed speech to the UK parliament in which he pleaded for Western jets.