The visit came as Russian President Vladimir Putin convened his security council in the wake of the latest spate of drone attacks on military-linked facilities inside Russian territory.

The focus of fighting in Ukraine has shifted to Donbas after Kyiv's forces recaptured the southern city of Kherson last month following a Russian retreat from the regional capital.

Zelensky appeared in a video wearing a heavy winter coat, standing next to a large sign in Ukraine's blue and yellow colours bearing the city name Sloviansk and calling for a moment of silence to commemorate killed Ukrainian soldiers.

"The east of Ukraine today is the most difficult front. And I am honoured to be here now with our defending troops in Donbas. I believe that next time we will meet in our Ukrainian Donetsk and Lugansk and in Crimea as well," Zelensky said.

Russian forces and their proxies have controlled parts of Donetsk and Lugansk since 2014, when fighting with separatists broke out and the Kremlin annexed the Crimean peninsula from Ukraine.

"From the bottom of my heart, I congratulate you on this great holiday, the Day of the Armed Forces," said Zelensky, who was later shown meeting soldiers and distributing awards.

In the nearby Russian-controlled city of Donetsk, its Moscow-appointed mayor said that Ukrainian shelling had killed six civilians and injured others.

- Drone attacks in Russia -

The Ukrainian leader has visited several frontline regions after more than nine months of fighting, including Kherson in the south recently recaptured by Ukrainian forces, calling its recapture "the beginning of the end of the war".

Sloviansk, which was among regions in Donetsk briefly held by Russian-backed separatists, lies some 45 kilometres (28 miles) north of Bakhmut, which has become the centre of fighting since Kherson's fall.

The Kremlin said Putin met senior officials to discuss issues related to the country's "domestic security" and that Russia was taking "necessary" measures to fend off more of what it said were Ukrainian attacks.

Officials in Russia's Kursk region near the border with Ukraine said earlier Tuesday that a drone attack near an airfield had caused a fire at a oil storage unit.

That attack came after the defence ministry said a day earlier that Ukraine had tried to attack another airfield in Ryazan region and also the key Engels airfield in the Saratov region.

Engels is a base for the country's strategic aircraft that Kyiv says have been used to strike Ukraine, and both sites are hundreds of kilometres away from Ukraine's border.

The British defence ministry said that if Russia deems Ukraine to have been responsible then Moscow will "probably consider them as some of the most strategically significant failures of force protection since its invasion of Ukraine."

The drone attacks come on the back of weeks of systematic Russian attacks that have crippled Ukrainian critical infrastructure like water, electricity and heating ahead of winter.

- 'Crush' Ukraine military -

Russia on Monday fired another barrage of dozens of missiles that knocked out power and water in cities across Ukraine, the latest wave of attacks that Moscow has said Kyiv was responsible for because it refused Russian demands.

Defence Minister Sergei Shoigu on Tuesday said that Russian forces were using long-range, precision weapons to target military-linked facilities and "crush the military potential of Ukraine".

The defence ministry also announced Tuesday it had received back from Ukraine captivity 60 Russian servicemen in their most recent exchange.

Russia's invasion and its decision to conscript hundreds of thousands of men has set off an exodus of Russians from the country, including critical politicians and journalists.

However, neighbouring Latvia announced Tuesday it was revoking the licence for exiled independent TV channel Dozhd for multiple violations that included showing the Crimea peninsula annexed from Ukraine as part of Russia.

"The laws of Latvia must be respected by everyone," Ivars Abolins, head of the Latvian National Electronic Mass Media Council, said on Twitter.