Ukraine is tightening up the defence of its border with Belarus over fears that Russia may be preparing a fresh attack, a government official has said.
Deputy Interior Minister Yevhen Yenin told the BBC Ukraine would be bolstering the Belarusian border with armed forces and ammunition.
The news came as Vladimir Putin travelled to Minsk to meet Belarusian leader Alexander Lukashenko.
Belarus shares a border with Russia as well as Ukraine.
The Russian president later ordered the strengthening of Russia's borders and of social control within Russia. He said the security services should quickly thwart any attempt to violate Russia's borders, combat risks coming from abroad, and identify traitors and saboteurs.
He also said the special services should ensure the safety of people living in the parts of Ukraine that Moscow claims as its own. In some of these areas, most notably Kherson, Russia recently suffered significant military reverses.
Russia's defence ministry announced that its troops stationed in Belarus would conduct joint military exercises with Belarus.
Reacting to this and the visit Mr Yenin confirmed: "We are building up our defence lines all across the border with Russia and with Belarus."
While Belarus has not become involved in the war directly, it did allow Russian troops to use its territory to launch the invasion in February.
Minsk is coming under increasing pressure from Moscow to step up its support in the "special military operation".
But Kremlin spokesman Dmitry Peskov dismissed the reports "as totally stupid, groundless fabrications".
President Putin flew to Minsk for talks with Mr Lukashenko – the first time in three-and-a-half years the pair have met in Belarus.
Belarus's authoritarian leader has been in power since 1994 but his re-election in 2020 has not been recognised by the EU.
The meeting between the two men has been described as a "working visit" and lasted for more than two hours.
Speaking at a joint press conference Vladimir Putin said Russia did not want to "absorb" anyone. He also said that unspecified "enemies" wanted to stop Russia's integration with Belarus.
Speaking later on Monday, US state department spokesperson Ned Price said Putin's statement was the "height of irony" given he was currently seeking to absorb Ukraine.
Something rare happened today: Vladimir Putin got on a plane and flew to Minsk. Now, President Putin and President Alexander Lukashenko meet a lot – but in Russia.
For the first time in three-and-a-half years, here was President Putin in Belarus. So why did he go… and why now?
There were few clues at the joint press conference as the two leaders spoke a lot about economic relations, trade, but also security.
President Putin indicated that some Belarusian military aircraft have been re-equipped to potentially carry nuclear missiles and that Russia is helping to train their crews.
In return, President Lukashenko thanked him for providing Belarus with an S-400 air defence system and Iskander ballistic missile system.
The word "Ukraine" was hardly mentioned – in public.
But the visit has fuelled speculation that the Kremlin leader may be trying to pressure the leader of Belarus into joining a possible new ground offensive in Ukraine.
This may indeed be just a rumour. Or a ploy by Moscow to make Ukraine expect a Belarusian offensive and so tie up Ukrainian soldiers in the north.
But, there are reportedly several thousand Russian soldiers already in Belarus and there have been joint exercises.
Until now Alexander Lukashenko has been reluctant to commit his own troops to fighting in Ukraine.
All eyes are on Minsk to see if that remains the case.
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