Pope Francis said Saturday he had only a chance meeting with Democratic presidential hopeful Bernie Sanders and advised anyone suggesting that he was meddling with US politics to "seek a psychiatrist."

"When I left this morning the senator was there. He knew that I would come out at that time and he waited to greet me. It was a greeting [...] no more," the pontiff told reporters on the plane back to Rome from the Greek island of Lesbos.

"If anyone thinks that greeting someone equates to meddling in politics, well, they should better seek a psychiatrist," Francis added.

The pope is wary of being exploited for political purposes, and usually avoids meetings with election candidates, mostly sticking to formal encounters with fellow heads of state.

Earlier, Sanders told US media he saw the pope at 6 am (0400 GMT) at his Vatican residence, exchanging views on climate change and foreign policy for five minutes before Francis left for Lesbos to meet with refugees.

The pope is "one of the great leaders in modern world history," Sanders told NBC News. "I am not a Catholic, but there is a radiance that comes from him. It was very wonderful to meet him," Sanders, who describes himself as a secular Jew, added.

The theme of economic inequality is central to Sanders' campaign and familiar territory for the pope, who has called it the "root of social evil."

Sanders travelled to the Vatican to address a Friday conference on social and economic issues at the Pontifical Academy of Social Sciences, but was originally told that a meeting with the pope was unlikely.

The Vermont senator trails rival Hillary Clinton in the delegate count for the Democratic presidential nomination. His trip to the Vatican came just days ahead of the New York primary election on April 19.

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