• Ukraine's president, Volodymyr Zelenskyy, has said he believes the next 24 hours will be a "crucial period" for his country.
• Ukrainian troops, and citizens who have taken up arms, continue to fight invading Russian forces.
• Russia has been hit with a massive round of sanctions isolating its economy and financial system.

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 who 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, Alexander 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.

Despite Lukashenko's saber-rattling, the country has offered to host peace talks between Ukrainian and Russian officials. Although the exact timing of discussions remains unclear, the Ukrainian president's office announced Monday that a delegation has arrived at Ukraine's border with Belarus to hold talks with Russian officials.

Zelenskyy said in a video Sunday that he had little hope of a breakthrough in discussions, but was willing to try, "I will say frankly, as always I do not really believe in the result of this meeting, but let them try."

What's the situation in Ukraine?

It's difficult to gauge the number of Ukrainian and Russian casualties in the conflict so far, given the chaotic and unpredictable nature of the fighting and emergency responses. Various reports from Ukraine have suggested that several hundred civilians have been killed, including a number of children. Ukraine has also claimed that several thousand Russian troops have been killed.

Russia has not revealed its number of casualties, but its Defense Ministry said on Sunday that "there have been killed and wounded among the Russian military," the BBC reported; Russia claimed its losses were "several times less" than the number of "exterminated" Ukrainians.

The U.K.'s Ministry of Defense issued an update on Monday which said "the bulk of Putin's ground forces remain more than 30km to the north of Kyiv their advance having been slowed by Ukrainian forces defending Hostomel airfield, a key Russian objective for day one of the conflict."

It also noted that "heavy fighting continues around Chernihiv and Kharkiv however both cities remain under Ukrainian control," and that, on a wider level, "logistical failures and staunch Ukrainian resistance continue to frustrate the Russian advance."

The West is ramping up the pressure on Russia in a bid to stop its invasion by imposing a raft of sanctions on Russia. The U.S., Europe and Canada agreeing Saturday to remove key Russian banks from the interbank messaging system, SWIFT, and sanctioned its central bank. The U.K. and EU have also closed their airspace to Russian aircraft, seeking to isolate Russia further.