MOSCOW -- The U.S. Embassy in Moscow urged American citizens in Russia to think about leaving the country immediately on Sunday, as some airlines halt flights and countries begin to close their skies to Russian aircraft.
"U.S. citizens should consider departing Russia immediately via commercial options still available," the Embassy said in a statement on its website.
U.S. officials in recent weeks have urged Americans not to travel to Russia, and warned that the U.S. government could not help in any evacuation of Americans from there.
An earlier alert recommended Americans develop contingency plans about how to leave the country if necessary.
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The European Union was among those announcing Sunday they were closing their airspace to Russian flights.
President Vladimir Putin dramatically escalated East-West tensions by ordering Russian nuclear forces put on high alert Sunday, while Ukraine's embattled leader agreed to talks with Moscow as Putin's troops and tanks drove deeper into the country, closing in around the capital.
Citing "aggressive statements" by NATO and tough financial sanctions, Putin issued a directive to increase the readiness of Russia's nuclear weapons, raising fears that the invasion of Ukraine could lead to nuclear war, whether by design or mistake.
The Russian leader is "potentially putting in play forces that, if there's a miscalculation, could make things much, much more dangerous," said a senior U.S. defense official, speaking on condition of anonymity to discuss rapidly unfolding military operations.
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Putin's directive came as Russian forces encountered strong resistance from Ukraine defenders. Despite Russian advances across the country, U.S. officials say they believe the invasion has been more difficult, and slower, than the Kremlin envisioned, though that could change as Moscow adapts.
Amid the mounting tensions, Western nations said they would tighten sanctions and buy and deliver weapons for Ukraine, including Stinger missiles for shooting down helicopters and other aircraft.
As of Sunday night, 352 people have been killed, including 14 children, according to Ukraine's Ministry of Internal Affairs.
They report 1,684 civilians have been wounded, 116 of which are children.
The Associated Press contributed to this report.