The warning from Ukraine to Belarus is blunt. "Your leadership is planning to drag the Belarusian people into a dirty war, to stain them with blood and death," the video released by the Ukrainian military declares. "If the Belarusian army supports Russian aggression, we will respond… with our entire arsenal of weapons." The warning comes as Russia is sending thousands of troops back to Belarus, prompting fears the two countries could be planning a joint incursion across Ukraine's northern border. That would be politically risky for Belarusian leader Alexander Lukashenko - and Russian forces are already struggling to hold their current lines around the southern city of Kherson and in Donbas in the east. But the prospect alone is a distraction and a concern for Kyiv as Minsk comes under pressure from Moscow to step-up its support. Wily operator "I think Putin really wants Belarusians to enter Ukraine so that Lukashenko… is bloodied, too, and will have to go to the end with him," is how Valery Sakhashchyk, the former commander of an elite paratrooper unit, reads the recent moves. Now in Warsaw, he is effectively the defence minister in a transitional cabinet of the Belarusian opposition-in-exile. The fuss was sparked when, after meeting Russian President Vladimir Putin earlier this month, Alexander Lukashenko claimed that Belarus's neighbours were planning to attack. He then announced that Minsk and Moscow were forming a joint "regional group" of forces for protection. Mr Sakhashchyk puts the risk of the two countries opening a northern front in Ukraine as one in three, currently. But he describes the long-time leader of Belarus as a "wily" operator. "Lukashenko will do everything possible not to send his troops to fight, to limit them to a supporting role, but there is certainly a threat."Belarus is already heavily involved in the Ukraine war. In February, Russian tanks crossed its southern border towards Kyiv and Russia regularly launches missiles from Belarusian territory. The EU is currently preparing another punishment package of sanctions in response. But if President Putin is pushing him for more, Mr Lukashenko has limited room for manoeuvre. The authoritarian leader has been dependent on Russia since 2020, when support from Moscow helped him survive an unprecedented wave of protests. Meanwhile, the mass imprisonment and torture of protestors hammered one nail in the coffin of relations with the West. Belarus aiding Russia's invasion was another. Mr Lukashenko says up to 9,000 Russian soldiers will come to Belarus for the new group. But sending Belarusians alongside them into Ukraine would be a deeply unpopular move. Several hundred Belarusians are already fighting in Ukraine - against Russia. Known as the Kalinovsky regiment, they openly say they joined the war to defeat Vladimir Putin so that Belarus, too, can be free. There is also resistance inside Belarus where partisans sabotaged railway lines at the start of the invasion to hinder the movement of Russian troops. Last week, another of the group was sentenced to 11 years in prison. "I get lads writing and saying we don't want to fight [against Ukraine]," Pavel Kukhta, of the Kalinovsky regiment, told me at their recruitment base in Warsaw. "My sources in the Belarussian army say 90% won't fight. They say the training's bad and kit and morale are even worse than the Russian army."