(CN) — The shelling of Ukrainian cities intensified on Monday as Ukraine and Russia exchanged accusations about war crimes and blamed each other for starting a war that has brought the world to the brink of a global catastrophe.

In Moscow, an increasingly ensnared and desperate Russian President Vladimir Putin is facing stark and potentially even more explosive decisions about what his next move will be, raising concerns that he may opt for further escalation rather than admit defeat.

“The fact that Putin is turning into a pariah on the international stage makes him even more dangerous and unpredictable,” said Andrius Tursa, an expert on Central and Eastern Europe at Teneo, a London-based political risk firm, in a briefing note.

Signs of Putin revving up his war machine were evident on Monday after Russia allegedly stepped up its missile campaign on Ukrainian cities. The loudest explosions yet heard struck Kyiv, the capital, and videos showed the potential use of cluster bombs in a residential area of Kharkiv, the country's second largest city. Media reported that seven civilians were killed and 44 people wounded.

Russia is coming under a crippling economic assault by Western powers that took a kind of shock-and-awe move to freeze the assets of Russia's central bank, a step that potentially leaves Putin unable to access about half of the $640 billion in reserves Russia has built up since the Ukraine crisis started in 2014 following the so-called “Maidan Revolution,” a U.S.-backed uprising that ousted Ukraine's democratically-elected pro-Russian president.

Putin’s invasion of Ukraine is advancing but meeting stiff opposition and Russian casualties and losses are mounting.

Ukraine claims more than 5,000 Russian soldiers have been killed, a number that cannot be verified. Russia has not said how many of its troops have been killed, but says that Ukrainian losses are far superior than its. Ukraine claims that about 300 soldiers have been killed.