Moscow: The US Embassy in Moscow has urged American citizens residing in Russia to leave immediately after the Russian government on September 21 began a mobilization of its citizens to the armed forces in support of its invasion of Ukraine.

The security alert was put out by the US Embassy in Russia on September 27.

"Russia may refuse to acknowledge dual nationals' US citizenship, deny their access to US consular assistance, prevent their departure from Russia, and conscript dual nationals for military service," the US Embassy wrote in a security alert for US Citizens in Russia.

It said that the commercial flight options are extremely limited at present and are often unavailable on short notice and added that the overland routes by car and bus are still open.

If you wish to depart Russia, the Embassy asked the US citizens in Russia to make "independent arrangements as soon as possible." The US Embassy noted that it has "severe limitations on its ability to assist US citizens, and conditions, including transportation options, may suddenly become even more limited."

"US citizens should not travel to Russia and those residing or travelling in Russia should depart Russia immediately while limited commercial travel options remain."

Moreover, the Embassy highlighted that the Department of State provides information on commercial travel on the Information for US Citizens in Russia - Travel Options Out of Russia page on travel.state.gov.

"This site also provides information on requirements for entering neighbouring countries, procedures for travel on expired US passports in some circumstances, and visa requirements for families with American and Russian citizen family members."

In a cautionary alert, the Embassy reminded US citizens that the "right to peaceful assembly and freedom of expression are not guaranteed in Russia."

"Avoid all political or social protests and do not photograph security personnel at these events. Russian authorities have arrested U.S. citizens who have participated in demonstrations."

Earlier on Friday, in a big move, Russian President Vladimir Putin formally announced the annexation of four regions - Donetsk, Luhansk, Kherson and Zaporizhia and claimed that "this is the will of millions of people," media reports said.

"There are four new regions of Russia," Putin said in a lengthy speech declaring the annexation of Ukrainian territories in the St George's Hall at a Kremlin ceremony. The speech was filled with highly critical rhetoric about Ukraine and its Western allies, reported Al Jazeera.

Putin said the residents in the four annexed regions will now be Russia's "citizens forever".

While denying seeking revival of the Soviet Union by the said annexation, Putin accused Western states - which have imposed sweeping sanctions on Moscow over its invasion of Ukraine.

Moreover, in a strong statement, Putin also added that Russia would now defend its new territory "with all the means at its disposal."

Defending its decision to go ahead with the referendum and announce the annexation of the region, the Russian President argued it was the "integral right" of people in Donetsk, Luhansk, Kherson and Zaporizhia to join Russia, reported Al Jazeera.

Speaking further, Putin claimed that the people in eastern Ukraine made up of Donetsk and Luhansk - had been "victims of inhumane terrorist attacks conducted by the Kyiv regime".

He also called the result of the referendum "the will of millions of people."

In a major move by Russia, President Vladimir Putin had signed a decree recognizing the so-called "independence" of the Ukrainian regions Zaporizhzhia and Kherson.

Moreover, the UN Security Council is set to vote Friday on a resolution that would seek to condemn Russia for its referenda in four regions under Russian control but claimed by Ukraine, media reports said.

UN Secretary-General Antonio Guterres said any annexation of a state's territory by another state resulting from a "threat or use of force" is a violation of the UN charter and international law.

On Thursday, US Secretary of State Antony Blinken accused Russia of the annexation of Ukrainian territories and called it illegal under international laws, media reports said. Describing Russia's move as an act of 'land grab', the top US diplomat said that the US will never recognise the annexation by Moscow.

European Union leaders condemned Russia's "illegal" annexation of Ukraine's Donetsk, Luhansk, Zaporizhzhia and Kherson regions and vowed that they will "never recognise" these areas.