What you need to know:
- Russia is facing an international backlash after Putin ordered his forces into Ukraine to secure two breakaway enclaves.
Former US president Donald Trump boasted of his close relationship with Russian leader Vladimir Putin on Tuesday, arguing that the Ukraine crisis would not have happened under his administration.
"If properly handled, there was absolutely no reason that the situation currently happening in Ukraine should have happened at all," he said in a statement.
"I know Vladimir Putin very well, and he would have never done during the Trump administration what he is doing now, no way!"
Russia is facing an international backlash after Putin ordered his forces into Ukraine to secure two breakaway enclaves.
The move came with tens of thousands of Russian soldiers on Ukraine's borders and amid warnings of an all-out invasion.
President Joe Biden imposed economic sanctions on the two enclaves, but on the question of penalties against Russia itself, a US official told reporters: "We are going to assess what Russia's done."
Biden will address the nation at 1:00 pm (1800 GMT) on Russia's actions, the White House said.
But Trump, who had been largely silent on the escalating Russian threat to the US ally, criticized the "weak" response which he said did not match Russia's actions.
"Now it has begun, oil prices are going higher and higher, and Putin is not only getting what he always wanted, but getting, because of the oil and gas surge, richer and richer," Trump added.
Former White House Russia advisor Fiona Hill told CNN Sunday that Trump's foreign policy had emboldened Putin.
The former president's foreign policy, she argued, had been driven by personal concerns rather than the national interest.
"There's no Team America for Trump. Not once did I see him do anything to put America first. Not once. Not for a single second," said Hill, one of Trump's most senior aides.
Ukraine found itself caught in a US domestic political firestorm when then-president Trump asked its leader to launch a spurious corruption probe into Joe Biden's family ahead of the 2020 presidential election.
Accusations that Trump tried to block military aid to pressure President Volodymyr Zelensky sparked the first of two impeachments of the Republican.