The United States embassy in Russia has advised its nationals to leave the country "immediately" warning them that commercial flights might not be available soon.

"An increasing number of airlines are canceling flights into and out of Russia, and numerous countries have closed their airspace to Russian airlines," the embassy said.

"US citizens should consider departing Russia immediately via commercial options still available," the alert on the embassy's website added.

The diplomatic mission also reminded its citizens that "the Department of State's Travel Advisory level for Russia is at 'Level 4: Do Not Travel.'"

This comes at a time of escalating fighting in Ukraine as part of Moscow's special military operation in the country over its security concerns.

Russia had for months been warning of the threat posed against it by NATO's attempts to expand eastward, which happened simultaneously with an increase in NATO military activity along Russia's borders, and batches of lethal weapons being sent to Ukraine, prompting Russia to request security guarantees from the West. Washington failed to provide the guarantees. 

After the West did not respond to Russia's demands, and amid Ukrainian shelling on the Donetsk and Lugansk People's Republics, President Vladimir Putin initiated a special military operation in Donbass. 

Moscow's operation conducted to prevent NATO from deploying troops along has been met with sanctions from a wide array of Western countries.

The European Union had agreed Friday to freeze European assets linked to Russian President Vladimir Putin and Foreign Minister Sergei Lavrov.

The European decision was accompanied by a wave of sanctions from the US, Europe, and other Western allies.