Stakes rose rapidly on Sunday (Feb 27) in Europe's biggest conflict since World War II after President Vladimir Putin put Russia's nuclear forces on high alert, an act that the United States termed an unacceptable escalation.
Mr Putin's decision came hours after Germany announced a historic shift in its defence policy to send arms to Ukraine in its fight against Russia and as Western nations put a squeeze on Moscow by booting some Russian banks from the Swift messaging system, used to facilitate transactions by banks and other financial institutions around the world.
Speaking alongside his defence minister and military chief, Mr Putin cited aggressive statements by leaders of US-led military alliance Nato and economic sanctions, as he ordered Russia's deterrence forces - a reference to its nuclear forces - on high alert.
"As you can see, not only do Western countries take unfriendly measures against our country in the economic dimension - I mean the illegal sanctions that everyone knows about very well - but also the top officials of leading Nato countries allow themselves to make aggressive statements with regard to our country," he said on national television.
He had made a reference to Russia's nuclear weapons, the largest stockpile in the world, in a speech on Thursday when he announced a "special military operation" against Ukraine, with a warning that any interference would result in consequences "you have never seen in your history".
While Mr Putin's order does not mean he is ordering Russia to prepare for a nuclear strike, it raised concern.
In a snap response, US Ambassador to the United Nations Linda Thomas-Greenfield said in TV interview: "President Putin is continuing to escalate this war in a manner that is totally unacceptable and we have to continue to stem his actions in the strongest possible way".
Nato's Secretary General Jens Stoltenberg described Mr Putin's statement as dangerous rhetoric.
The military alliance has beefed up forces on its eastern flank but so far, it has ruled out deploying troops to fight Russia.
On the fourth day of the invasion, Ukraine held its ground in the face of intensified combat operations and aerial bombardment of its capital Kyiv and many other cities. Russian forces entered the second-largest city of Kharkiv and blew up a gas pipeline, sparking huge explosions. The city's leaders later said Ukrainian fighters had repulsed the attack.
Authorities warned that Russia would continue to cause "man-made disasters" by assaulting chemical plants and critical infrastructure and asked residents to close their windows and take other safety precautions.
US officials said that most of the more than 150,000 Russian troops which had been stationed around Ukraine were now engaged in battle.
While asking citizens to keep fighting, President Volodymyr Zelensky also said he would hold talks without preconditions with Russia at its border with Belarus after he received assurances that "all planes, helicopters and missiles stationed on Belarus territory will remain on the ground during the travel, negotiations and return of the Ukrainian delegation".
Details of exactly when the meeting would take place or who would participate were not immediately clear.
The United Nations has confirmed at least 240 civilian casualties since the invasion began but the real figures are likely to be considerably higher. Thousands of women and children continued to flee into neighbouring countries, prompting a plead from Pope Francis for humanitarian corridors.
At least 100,000 people are displaced internally in Ukraine while the numbers crossing into the European Union stood at more than 350,000. EU officials had forecast more than a million refugees in the event of war.
As Russia pushed ahead with its offensive to overthrow Ukraine's government and recreate its Cold War-era influence, Western nations stepped up commitments to equip the outnumbered Ukrainian forces with weapons and ammunition. The White House has asked Congress to authorise US$6.4 billion (S$8.7 billion) in emergency aid for Ukraine, which would be divided between humanitarian and military aid.
Germany, the largest economy in the European Union, will send 1,000 anti-tank weapons and 500 Stinger missiles to Ukraine in a significant reversal of its restrictive arms export policy.
"There could be no other answer to Putin's aggression," Chancellor Olaf Scholz said during a special parliamentary session.
Italy will match the pledge made by Germany while the Czech Republic dispatched a train to Ukraine carrying machine guns, assault rifles and other weaponry. Romania, Poland, Greece and Portugal also announced military aid. Most European nations and Canada also shut their airspace to Russian aircraft.
Along with the decision to cut off some Russian banks from Swift messaging service, the US and European nation imposed restrictions on Russia's Central Bank and limited the sale of "golden passports" that allow wealthy Russians to become citizens of Western countries. They also beefed up coordination against disinformation and other forms of hybrid warfare.
The measures are intended to undermine Kremlin's ability to use its over US$630 billion of central bank reserves to blunt the impact of sanctions.