Police officers stand guard at a checkpoint on February 28, 2022 in Kyiv, Ukraine on Feb. 28, 2022. (Photo by Chris McGrath/Getty Images)

WASHINGTON: The United States has not seen any “noticeable muscle movements” or changes to Russia’s nuclear posture, a senior defense official said today, following Russian President Vladimir Putin’s order on Sunday to put Russia’s nuclear forces on “special combat readiness” alert.

“I don’t believe we’ve seen anything specific as a result of the direction that he gave, at least not yet,” the official told reporters during a briefing. Because the phrase used by Putin isn’t associated with known Russian doctrine, the US is “still analyzing it and reviewing it to try to understand what exactly it means.”

Putin cited NATO sanctions and condemnation as the reason for putting Russia’s strategic forces on heightened alert, stating that “top officials from leading NATO members [have] made aggressive statements regarding our country,” according to CBS News.

White House spokeswoman Jen Psaki told ABC News on Sunday called Putin’s statements highly escalatory, adding that the Russian president was “manufacturing threats that don’t exist in order to justify further aggression.”

Meanwhile, Russia’s invasion of Ukraine continues to proceed more slowly than it had projected, despite the fact that Putin has now committed almost three-quarters of the forces it previously staged on Ukraine’s borders and launched more than 380 missiles since the conflict began on Thursday, the defense official said.

The Pentagon estimates that Russian forces are about 15 miles from Ukraine’s capital city of Kyiv — with ground troops making only about 3 miles worth of progress Sunday, the official said.

“We expect that they’re going to want to continue to move forward and try to encircle the city,” the officials said.

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Russian forces have also continued to try to overtake the cities of Kharkiv and Mariupol, which would allow Russia to hold back Ukrainian troops located in the eastern portion of Ukraine, the official said.

However, Ukraine’s success in staving off Russia’s offense “could result in a reevaluation of [Russian] tactics and the potential for them to be more aggressive and more overt in both the size and scale of their targeting of Kyiv,” the official said. “We haven’t seen that yet, but we’re certainly mindful that the frustrations could lead to a more aggressive approach by the Russians.”

Despite Russian claims that it had established air superiority over the skies of Ukraine, the official said Ukraine’s airspace remains contested, with Ukrainian air defense systems and aircraft still operational.

“The Ukrainians are proven very creative in how they’re resisting, you know, blowing up bridges for instance and attacking them [Russian troops] in small numbers, sometimes lightly armed,” the official said. “They’re using pretty much everything that they have in their arsenal from small arms all the way up to surface to air missiles to try to slow down the Russians.”

Although Ukrainian troops have had to defend the country from Russia’s massive military without the direct involvement of NATO combat forces, the US and several other European nations pledged additional military assistance to Ukraine over the weekend.

On Friday, the US authorized $350 million for “lethal defensive assistance to help Ukraine address the armored, airborne, and other threats it is now facing,” according to a statement by Secretary of State Antony Blinken. The assistance will include anti-armor, small arms and various munitions, body armor, and related equipment, according to the Pentagon.

A day later, Germany announced its own wave of aid, breaking with its longstanding policy prohibiting weapons to be transferred to a combat zone. According to Politico, the aid includes 1,000 anti-tank weapons and 500 Stinger anti-aircraft defense systems, as well as permission for Netherlands and Estonia to send German-made howitzers and rocket-propelled grenade launchers.

Then, in another unprecedented move, the European Union on Sunday offered approximately $500 million in lethal military assistance and an additional $56 million for nonlethal supplies, fuel and protective gear. The EU also announced several steps meant to put pressure on Russia, including blocking Russian aircraft from flying in EU airspace and banned Russian propaganda outlets Russia Today and Sputnik.

Ukraine is also seeking out additional fighter jets from EU countries that have flown Sukhoi and MiG aircraft similar to the models currently operated by Ukraine, EU foreign policy chief Josep Borrell told reporters on Sunday, according to the Wall Street Journal.