The US embassy in Moscow on Sunday warned Americans of potential attacks in public places in Russia, including along the border with Ukraine.

"According to media sources, there have been threats of attacks against shopping centers, railway and metro stations, and other public gathering places in major urban areas," the embassy said in statement quoted by AFP.

The mission did not point to specific reports.

The embassy's warning said there was a risk of attacks in Moscow and Saint Petersburg, "as well as in areas of heightened tension along the Russian border with Ukraine".

The embassy told Americans in Russia to "avoid crowds" and "have evacuation plans that do not rely on US government assistance".

Russia's foreign ministry spokeswoman Maria Zakharova criticized the move on social media, asking whether the US side had followed protocol with the announcement and asking: "What are we to make of this?"

The warning comes hours after the US cited intelligence reports saying that Russian military officers have already received orders to launch the invasion of Ukraine.

Russian commanders are now “doing everything that American commanders would do once they got the order to proceed,” said CBS News national security correspondent David Martin, who confirmed the veracity of the intelligence reports.

The Biden administration has yet to entirely despair of averting full-scale war, but hopes are not high.

Speaking on CNN’s “State of the Union,” Secretary of State Antony Blinken said that Russian president Vladimir Putin’s positioning of forces so close to the Ukrainian border indicated an impending incursion.

“He’s followed the script almost to the letter,” Blinken said. “We believe President Putin has made the decision, but until the tanks are actually rolling, and the planes are flying, we will use every opportunity and every minute we have to see if diplomacy can still dissuade President Putin from carrying this forward,” he added.

US President Joe Biden told reporters on Thursday that there are indications that Russia still intends to invade Ukraine, and announced that he does not intend to talk at this stage with Russian President Vladimir Putin.

The President added that an invasion may occur in the next several days.

On Saturday, British Prime Minister Boris Johnson said that evidence suggests Russia is planning "the biggest war in Europe since 1945".

"All the signs are that the plan has already in some senses begun," Johnson told the BBC's Sophie Raworth.

Intelligence suggests Russia intends to launch an invasion that will encircle Ukrainian capital Kyiv, Johnson said.

The comments came hours after Ukrainian President Volodymyr Zelensky urged allies to begin sanctioning Russia now, rather than wait for an invasion.

Noting the repeated American declarations that an attack on his country would occur within days, Zelensky asked, “What are you waiting for?’’