Bush and Clinton leave flowers at Ukrainian Village church in Chicago

In a display of solidarity with Ukraine, George W Bush and Bill Clinton earlier this week left bouquets of sunflowers, the war-torn nation's national flower, at an iconic church in Chicago's Ukrainian Village during an unannounced visit.

Both former US presidents, who have been outspoken in opposing Russia's invasion of Ukraine, used social media late Friday to post videos of their joint visit to the golden-domed Saints Volodymyr and Olha Ukrainian Catholic Church.

The men, both 75, wore yellow ribbons on their suit lapels as they approached the church, carrying the bouquets of yellow sunflowers tied in a blue ribbon. They left the flowers, which came from a local florist, at a stone cross outside the church.

"America stands in solidarity with the people of Ukraine as they fight for their freedom and their future," Bush, a former Republican president, said on Twitter through his presidential centre in Dallas.

Clinton, a former Democratic president, posted the video to his Twitter page, writing, "America stands united with the people of Ukraine in their fight for freedom and against oppression."

In the video, an unidentified church member explained that the flowers say "in solidarity" and Ukraine has become "the citadel of fighting for freedom."

The video said Clinton and Bush "worked to support Ukraine's democratic institutions" after the United States began diplomatic relations with the country in 1991 following the fall of the Soviet Union.

In a Facebook post, the church displayed the video and thanked the former presidents for their visit.

"Two former presidents of the United States arrived to our Cathedral to express support for Ukraine. Thank you Mister President Bush and Mister President Clinton," the Facebook post said.

Both Bush and Clinton have criticized Russian President Vladimir Putin's invasion of Ukraine, which began on February 24.

In a statement following Russia's invasion, Bush called it "unprovoked and unjustified" and said it constitutes "the gravest security crisis on the European continent since World War II."

Clinton also has called Putin's actions "unprovoked and unjustified" and warned Putin that "the world will hold Russia, and Russia alone, accountable, both economically and politically."