Russia launched a far-reaching military attack on Ukraine on Thursday, invading the country through multiple border sites and bombing more than a dozen cities as civilians attempted to flee their homes in panic.

One Ukrainian official reported that "hundreds" have likely been killed thus far as explosions and artillery fire were heard throughout the country, including in the capital Kyiv. Ukraine's military said it shot down six Russian warplanes and one helicopter, but Russia denied that any of its aircraft were struck.

"We will defend ourselves," Ukrainian President Volodymyr Zelensky said in a speech ahead of the invasion. Zelensky has since imposed martial law, noting that Russia "conducted strikes on our military infrastructure and our border guards."

Russia's assault—which came after months of troop build-ups along the Ukrainian border—drew widespread international condemnation, with world leaders and humanitarian groups denouncing the invasion as an illegal act of aggression and demanding an immediate withdrawal of Russian forces.

"In the name of humanity, bring your troops back to Russia," United Nations Secretary-General António Guterres said Thursday in a direct appeal to Russian President Vladimir Putin. "In the name of humanity, do not allow to start in Europe what could be the worst war since the beginning of the [20th] Century."

"It violates the principles of the [U.N.] Charter," Guterres added. "And it will cause, if it doesn't stop, a level of suffering Europe has not known since, at least, the Balkan crisis."

The West is expected to respond to Russia's attack on Ukraine with major sanctions on top of the punitive measures imposed in the wake of Putin's deployment of forces into breakaway regions of eastern Ukraine earlier this week.

"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," U.S. President Joe Biden said in a statement. "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."

The Spanish government also released a statement calling for "an immediate cease to hostilities before the number of victims rises, and for the return of troops to the internationally recognized Russian Federation territory."

In a televised speech before publicly authorizing the invasion—which he described as a "special military operation"—Putin delivered a sharp warning to the West and NATO.
"Whoever tries to impede us, let alone create threats for our country and its people, must know that the Russian response will be immediate and lead to the consequences you have never seen in history," said Putin, who has cited Ukraine's ambition to join NATO as well as the positioning of U.S. weaponry in Eastern Europe as major security threats to Russia.

"No one should have any doubts that a direct attack on our country will lead to the destruction and horrible consequences for any potential aggressor," Putin added.
While Biden has said he has no intention of sending U.S. troops to directly fight Russian forces in Ukraine, the U.S. has deployed thousands of soldiers to Europe in recent weeks in preparation for a possible attack.

Anti-war campaigners have said repeatedly in recent weeks that the dangers of a war in Ukraine—particularly one directly involving the U.S. and Russia, which together possess more than 90% of the world's nuclear arsenal—cannot be overstated.

Ukraine does not possess nuclear weapons, but it does maintain more than a dozen nuclear power reactors that could come under threat during the ongoing attack.

"No matter the genesis, the cause, or who started what, the reality remains that there are 15 operating nuclear reactors in Ukraine that, if conflict breaks out there, could be in peril," Linda Pentz Gunter, founder of the advocacy group Beyond Nuclear, said Tuesday. "If the reactors find themselves amidst a conflict or war, they cannot simply be abandoned by the workforce. This makes the prospects of a war in Ukraine all the more alarming, and the imperative to avoid this all the more urgent."

In a statement early Thursday, the U.S.-based progressive advocacy group RootsAction said that "the latest actions by the Russian government will cause death and suffering, and those actions will further destabilize global security while rendering institutions like the United Nations even more powerless."

"As always, in this instance the aggressor has couched its aggression by claiming to act in defense, as the U.S. government often has," the group added. "At this extremely dangerous moment in world history, diplomacy—not warfare —remains the only hope. In the process, the perilous history of NATO's eastward expansion and the threat of further expansion must be faced. Fueling a conflict between the two nuclear superpowers is insanity."