Russia's offensive in eastern Ukraine was gaining speed and seizing more ground on Wednesday ahead of a summit in Alaska between US President Donald Trump and Russia's Vladimir Putin.

According to battlefield data from the US-based Institute for the Study of War, Russian forces have made their biggest 24-hour advance into Ukraine in more than a year.

And, as battle raged, Ukraine's President Volodymyr Zelensky and European leaders met to hold urgent online talks with Trump, hoping to convince him to respect Kyiv's interests during Friday's summit with Putin.

Zelensky flew to Berlin and met Chancellor Friedrich Merz on Wednesday before both were to be joined online by French President Emmanuel Macron, British Prime Minister Keir Starmer and other European leaders as well as the heads of the EU and Nato.

The Ukrainian president said on social media on Tuesday that "we see that the Russian army is not preparing to end the war. On the contrary, they are making movements that indicate preparations for new offensive operations."

With the world's eyes on the Alaska summit, Russia has made rapid advances this week in a narrow but important section of the front line in Ukraine's east.

The Russian military fired at least 49 drones and two ballistic missiles at Ukraine during the night, the Ukrainian air force said on Wednesday.

Trump on Monday played down the possibility of a breakthrough in Alaska but said he expected "constructive conversations" with Putin. (AFP)