Ukraine on Monday hailed its first substantial battlefield advances for six months as President Volodymyr Zelensky won pledges of new long-range drones in Britain to add to a haul of western arms for a counteroffensive against Russian invaders.

Since last week, the Ukrainian military has started to push Russian forces back in and around the battlefield city of Bakhmut, its first significant offensive operation since its troops recaptured the southern city of Kherson in November.

"The advance of our troops along the Bakhmut direction is the first success of offensive actions in the defence of Bakhmut," Colonel General Oleksandr Syrskyi, Commander of Ground Forces, said in a statement on the Telegram messaging app.

"The last few days have shown that we can move forward and destroy the enemy even in such extremely difficult conditions," he said. "We are fighting with fewer resources than the enemy. At the same time, we are able to ruin its plans."

The battle for the small eastern city has become the longest and bloodiest of the war and has totemic significance for Russia, which has no other prizes to show for a winter campaign that cost thousands of lives.

Over the past half of a year, Kyiv has held its troops on the defensive while Moscow mounted its campaign, sending hundreds of thousands of fresh reservists and mercenaries into Europe's bloodiest ground combat since World War II.

Kyiv is now preparing a counteroffensive using hundreds of new tanks and armoured vehicles sent by western countries since the start of this year, aiming to recapture the sixth of Ukraine's territory that Moscow claims to have annexed.

Zelensky met British Prime Minister Rishi Sunak in London on Monday, the latest stop in a tour that brought him to Rome, Berlin and Paris over the past three days, pocketing major new pledges of weapons along the way.

Britain, which last week became the first western country to offer Ukraine long-range cruise missiles, followed that up during Zelensky's visit on Monday with a pledge of drones that could strike at a range of 200 km.

Sunak's government also said it would soon start training Ukrainian pilots to fly fighter jets, although it would not provide planes for Ukraine as Kyiv wants US F-16s which Britain's air force does not use.

Zelensky described the new weapons pledged by the Europeans as "important and powerful". The priority during his talks has been "our counteroffensive actions. I am very pleased with the achievements and agreements", he said.

Sunak said the war was at a "pivotal moment".