When I saw her, it was genuine expression of affection: Bush on Michelle

Former Republican President George W. Bush has opened up about his friendship with former Democratic First Lady Michelle Obama, who are surprisingly close considering their political differences.

According to CNN, the 43rd President credits Michelle Obama's appreciation for his sense of humour as a key reason.

They have often been photographed together during formal events.

The photos of their warm interaction went viral when Obama embraced Bush at the opening ceremony for the Smithsonian National Museum of African American History and Culture last September.

"When I saw her, it was a genuine expression of affection," the former Republican President said on Wednesday as he reflected on the moment.

Bush is back in the public eye to promote his new book, Portraits of Courage, a volume of his paintings of military veterans.

The Bushes and the Obamas join hands during the singing of The Battle Hymn of the Republic during an interfaith memorial service for the victims of Dallas police shooting on July 12 last year.

"She kind of likes my sense of humour. Anybody who likes my sense of humour, I immediately like," Bush said of Michelle Obama.

"(The friendship) surprised everybody. That's what's so weird about society today, (the surprise) that people on opposite sides of the political spectrum can actually like each other."

The pair are often seated next to each other during events, including the interfaith memorial service, last year's funeral for former First Lady Nancy Reagan and the 2015 commemoration of the 50th anniversary of the historic civil rights march in Selma, Alabama.

"I can't remember where else I've sat next to her, but I probably have a few wise-cracks and she seemed to like it OK," Bush said.

"I needle her a little bit and around her, I'm fairly lighthearted. (The Obamas) are around serious people all the time and we just took to each other."

Most recently, the Bushes attended President Donald Trump's inauguration on January 20, where they were seen chatting with the Obamas.