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. defence official, speaking on condition of anonymity to discuss rapidly unfolding military operations.

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.