American citizens were urged to leave Russia immediately in a security alert issued by the U.S. Embassy in Moscow due to a security alert overnight, CNN reports.

The alert comes one week after Russian President Vladimir Putin announced a partial military mobilization of 300,000 reservists and reiterated warnings to Western allies that he was ready to use nuclear weapons in Moscow's ongoing invasion of neighboring Ukraine, during a rare, prerecorded television announcement on September 21.

“Russia may refuse to acknowledge dual nationals’ U.S. citizenship, deny their access to US consular assistance, prevent their departure from Russia, and conscript dual nationals for military service,” the U.S. Embassy alert said on Tuesday (September 28).

"Russian authorities have arrested U.S. citizens who have participated in demonstrations,” the alert added.

American citizens in Russia were urged by the embassy to "avoid all political or social protests and do not photograph security personnel at these events" and the alert acknowledged “the right to peaceful assembly and freedom of expression are not guaranteed in Russia.”

Ukraine and its Western allies said Putin's call for more troops signified that his invasion of the neighboring country was failing in response to the announcement last week.

Allies vowed to continue their support of Ukraine amid Putin's announcement.

Putin announced his country would conduct military operations in Eastern Ukraine during an NBC News translation of a speech addressing the Russian population in Moscow on February 24.

The announcement appeared to serve as the final action ahead of an attack by Putin and the Russian military, which the U.S. and European allies to the neighboring Ukraine have attempted to prevent from taking place through diplomatic discussions.

A Ukraine interior minister confirmed to NBC News via text message that "cruise and ballistic missile strikes" were already underway shortly after Putin's announcement.

NBC News correspondent Erin McLaughlin said explosions could be heard from her live shot in Kyiv, Ukraine's capital city, at 6:00 a.m. local time.

More than 1,000 protesters were reportedly arrested during anti-war protests throughout Russia amid President Putin's announcement to conduct military operations and ensuing attack on Ukraine, BNO News reported.