Russian President Vladimir Putin announced a "special military operation" in Ukraine on Thursday, local time.
Putin said in an emergency address on Russian TV that the military operation is intended to protect civilians and to demilitarize Ukraine, and is a response to threats coming from Ukraine.
He said that Russia doesn't plan to occupy Ukraine and said that the Ukrainian military should "immediately lay down its arms."
Putin also said that any foreign attempt to interfere with Russia's action will trigger an immediate reaction from Russia, and will lead to "consequences they have not seen before."
Putin's announcement comes after he announced Monday that Russia recognizes the Luhansk and Donetsk regions in eastern Ukraine as independent, and signed a decree to authorize the Russian Defense Ministry to send troops into the area for "peacekeeping operations."
"The prayers of the entire world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack by Russian military forces," Biden said. "Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way. The world will hold Russia accountable."
Biden said he will monitor the situation from the White House in the evening and get regular updates from the national security team.
"Tomorrow, I will meet with my G7 counterparts in the morning and then speak to the American people to announce the further consequences the United States and our Allies and partners will impose on Russia for this needless act of aggression against Ukraine and global peace and security," Biden said. "We will also coordinate with our NATO Allies to ensure a strong, united response that deters any aggression against the Alliance. Tonight, Jill and I are praying for the brave and proud people of Ukraine."
Former President Donald Trump called into Fox News, telling host Laura Ingraham, "This should have never happened. It would have never happened under my administration. ... It's a very sad thing for the world, for the country, and it's certainly very sad for a lot of people who are going to be needlessly killed."
"The world can and must stop Putin. The time to act is now."
He called the situation "a grave breach of international law, and a serious threat to Euro-Atlantic security."
"I call on Russia to cease its military action immediately and respect Ukraine’s sovereignty and territorial integrity," Stoltenberg said. "NATO Allies will meet to address the consequences of Russia’s aggressive actions. We stand with the people of Ukraine at this terrible time. NATO will do all it takes to protect and defend all Allies."
Guterres was attending an emergency meeting of the U.N. Security Council, called by Ukraine, just prior to Putin's announcement.
"President Putin, in the name of humanity, bring your troops back to Russia," Guterres said. "In the name of humanity, do not allow to start in Europe what could be the worst war since the beginning of the century."