Ukraine is urging people living in some eastern regions to flee immediately as Russia focuses its military efforts in that part of the country after pulling out of the north.

Russian troops have been amassing in eastern Ukraine and it’s feared they are planning another offensive after reorganising the military leadership.

About one-third of citizens who had opted to remain in the Luhansk region have been told to evacuate as Russia increases its shelling.

As air-raid sirens sounded in cities across eastern Ukraine, President Volodymyr Zelensky said the country was ready for a tough battle with the Russian forces.

It comes a day after Russians hit a train station crowded with women, children and the elderly in the eastern city of Kramatorsk.

“Yes, (Russian) forces are gathering in the east (of Ukraine),” Mr Zelensky told a joint news conference with Austrian Chancellor Karl Nehammer in Kyiv.

“This will be a hard battle, we believe in this fight and our victory. We are ready to simultaneously fight and look for diplomatic ways to put an end to this war.”

The Ukrainian military says Moscow is preparing for a thrust to try to gain full control of the Donbas regions of Donetsk and Luhansk that have been partly held by Moscow-backed separatists since 2014.

Air attacks are likely to increase in the south and east as Russia seeks to establish a land bridge between Crimea — which Moscow annexed in 2014 — and the Donbas.

Meanwhile British Prime Minister Boris Johnson was in Kyiv and met with Mr Zelensky on Saturday “in a show of solidarity with the Ukrainian people”, a Downing Street spokesperson said.

“The UK will send more defensive weapons to Ukraine and will work with G7 partners to target every pillar of the Russian economy to ensure Putin fails,” Mr Johnson tweeted earlier on Saturday.

Mr Zelensky said the UK was at the forefront of international support for Ukraine.

Mr Johnson and Austria’s chancellor visited a day after European Commission President Ursula von der Leyen.

In another such move, Italy said it would re-open its embassy in Kyiv after Easter.

A woman at makeshift grave sites in the Donbas region where the Russians are focussing their troops. Photo: AAP

Friday’s missile attack on the train station in Kramatorsk, a hub for civilians fleeing the east, left shreds of blood-stained clothes, toys and damaged luggage strewn across the station’s platform.

City Mayor Oleksander Honcharenko, who estimated 4,000 people were gathered there at the time, said on Saturday that the death toll had risen to least 52.

Russia’s defence ministry denied responsibility, saying in a statement the missiles that struck the station were used only by Ukraine’s military and that Russia’s armed forces had no targets assigned in Kramatorsk on Friday.

Russian state television described the attack as a “bloody provocation” by Ukraine.

In Washington, a senior defence official said the United States did not accept the Russian denial and believed Russian forces had fired a short-range ballistic missile in the attack.

The other developments in Ukraine:

• British military intelligence said Russian operations continue to focus on the Donbas region, Mariupol and Mykolaiv, supported by continued cruise missile launches by Russian naval forces.
• Russian forces have destroyed an ammunition depot at the Myrhorod Air Base in central-eastern Ukraine, Interfax news agency reported, quoting Russia’s defence ministry.
• The Kremlin said on Friday the “special operation” in Ukraine could end in the “foreseeable future” with its aims being achieved by the Russian military and peace negotiators.
• NATO Secretary-General Jens Stoltenberg warned of a war that could last months or even years.
• Ten humanitarian corridors to evacuate people from besieged regions have been agreed for Saturday, Deputy Prime Minister Iryna Vereshchuk said.
• Britain added Vladimir Putin’s daughters to its sanctions list, mirroring moves by the United States, in what it said was an effort to target the lifestyles of those in the Russian president’s inner circle.
• The United States on Friday broadened its export curbs against Russia and Belarus, restricting access to imports of items such as fertiliser and pipe valves as it seeks to ratchet up pressure on Moscow and Minsk.

-with AAP