President Vladimir Putin has ordered Russian nuclear deterrent forces on alert amid tensions with the West over his invasion of Ukraine.

Speaking at a meeting with his top officials, Putin asserted on Sunday that leading NATO powers had made “aggressive statements” along with the West imposing hard-hitting financial sanctions against Russia, including the president himself.

Putin ordered the Russian Defence Minister and the chief of the military's General Staff to put the nuclear deterrent forces in a “special regime of combat duty.”

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His order raised the threat that the tensions with the West over the invasion in Ukraine could lead to the use of nuclear weapons.

The Russian leader this week threatened to retaliate harshly against any nations that intervened directly in the conflict in Ukraine.

The capital, Kyiv, was eerily quiet after huge explosions lit up the morning sky and authorities reported blasts at one of the airports.

Only an occasional car appeared on a deserted main boulevard as a strict 39-hour curfew kept people off the streets. Terrified residents instead hunkered down in homes, underground garages and subway stations in anticipation of a full-scale Russian assault.

Following its gains to the east in the city of Kharkiv and multiple ports, Russia sent a delegation to Belarus for peace talks with Ukraine, according to the Kremlin. Zelenskyy suggested other locations, saying his country was unwilling to meet in Belarus because it served as a staging ground for the invasion.

Until Sunday, Russia's troops had remained on the outskirts of Kharkiv, a city of 1.4 million about 20 kilometers (12.4 miles) south of the border with Russia, while other forces rolled past to press the offensive deeper into Ukraine.