Ukraine’s President Volodymyr Zelenskyy has said he believes the next 24 hours will be a “crucial period” for his country as Ukrainian troops — and citizens that have taken up arms — continue to fight invading Russian forces.Zelenskyy made the comment when speaking to his British counterpart, Boris Johnson, on Sunday evening, after Johnson praised the bravery of the Ukrainian people. He also commended Zelenskyy’s leadership “in the face of such adversity,” the British government said in a statement.Ukraine’s major cities remain under attack from Russian forces invading from the north, east and south. Assaults are taking place both on the ground — with street fighting reported in a number of cities — and from the air, with major shelling attacks damaging buildings and causing injuries and loss of life in some cases.Overnight, blasts were heard in the Ukrainian city of Kharkiv and the capital Kyiv, ahead of the city lifting a two-day curfew on Monday morning allowing anyone remaining in the city to venture outside.Ukraine’s armed forces and volunteer fighters have mounted a strong defense against the invasion, with multiple reports saying that Russia’s forces have not made the advances they expected since the attack began early last Thursday.The Belarus factor?Nonetheless, Ukraine’s resistance may soon come under increased pressure, given fears that Belarus could deploy its own troops to support Russia. CNBC has contacted the Belarusian defense ministry for comment.Belarus has also signaled that it could host Russian nuclear weapons after a referendum in the country approved constitutional reforms that will see the country renounce its non-nuclear status.The move is widely seen as another step in Belarus’ backing of its ally Russia and comes after Russian President Vladimir Putin put his country’s nuclear deterrence forces on high alert Sunday amid a growing global backlash against the invasion.Belarus’ President Lukashenko warned on Sunday: “If you [the West] transfer nuclear weapons to Poland or Lithuania, to our borders, then I will turn to Putin to return the nuclear weapons that I gave away without any conditions,” Lukashenko said.cnbc.com