A few days ago, Zelensky said his country was ready to launch the long-awaited counteroffensive to retake Russian-occupied territory, adding: "We strongly believe that we will succeed."

"I don't know how long it will take. To be honest, it can go in many completely different ways. But we will do it, and we are ready."

A Ukrainian military spokesman said on Saturday Ukrainian forces had advanced as part of a counteroffensive up to 1400,<> meters in a number of frontline locations near the eastern city of Bakhamout on Friday.

Serhii Shirevati, spokesman for the Eastern Military Command, said in response to a question about the fighting near Pakhmut: "We are trying to launch strikes on the enemy, we are conducting a counterattack, we have managed to advance up to 1400,<> meters in several positions of the front."

"Russian forces themselves were trying to launch a counterattack but were unsuccessful," Sherevati said in remarks.

Moscow and Kiev said heavy fighting broke out in Ukraine on Friday, and bloggers reported seeing German and U.S. armored vehicles for the first time, a sign that a long-anticipated Ukrainian counteroffensive may have begun.

Russia, which has set up large-scale fortifications in areas it controlled in eastern and southern Ukraine, said last week that Kiev had tried hard to break Russian lines but failed.

Britain's Ministry of Defence said Ukrainian forces had breached the first line of Russian defences in some areas, but Kiev's advance had been slower in others.

Officials in Ukraine, which has been preparing for a large-scale counteroffensive for weeks, deny the long-awaited move has begun and say that when it starts it will be clear.