Hillary Clinton said that tactics implemented by Russia in Crimea are reminiscent of those used by Nazi Germany before World War II.
Speaking at a fundraiser in California, Clinton drew a parallel between Adolf Hitler's rhetoric that ethnic Germans living in neighbouring countries he later invaded were oppressed, and Vladimir Putin's claim that ethnic Russians in Ukraine are under threat by nationalists and radicals.
The former US secretary of State, who is widely expected to run for US president in 2016, particularly criticised Moscow over the issuing of passports to ethnic Russians living in Ukraine's Crimean peninsula.
"Now if this sounds familiar, it's what Hitler did back in the 30s," Clinton said, the Long Beach Press-Telegram reported.
"All the Germans that were ... the ethnic Germans, the Germans by ancestry who were in places like Czechoslovakia and Romania and other places, Hitler kept saying they're not being treated right. I must go and protect my people and that's what's gotten everybody so nervous."
Clinton, 66, made the comment at a $1,500-a-head fundraiser for the Boys and Girls Club of Long Beach.
She also said that Putin believes "his mission is to restore Russian greatness."
"When he looks at Ukraine, he sees a place that he believes is by its very nature part of Mother Russia," Clinton said.
Harry Saltzgaver, who attended the event, told Buzzfeed that Clinton added there was "no indication that Putin is as irrational as the instigator of World War II."
Earlier this week, Vladimir Putin was accused of acting like Hitler in 1930s also by the former foreign minister of the Czech Republic, Karel Schwarzenberg.
"Since he wanted to invade Crimea, he needed a pretext and said that his compatriots were oppressed," said Schwarzenberg."When Hitler wanted to annex Austria, he said that Germans there were oppressed."
Putin has claimed it would be legitimate for Russia to use force to protect its interests in eastern Ukraine and Crimea.
"We have received a request from a legitimate president," Putin told a press conference. "Also we have historical and cultural ties with those people. And this is a humanitarian mission. It's not our goal to conquer somebody."
He said deploying the military would be the last resort but and denied that troops without insignia who took over strategic locations in Crimea are Russian.