Pope Francis said on Saturday that his meeting with Bernie Sanders, contesting the Democratic candidacy for the U.S. Presidency, was not meddling in politics and that anyone who thought otherwise should "look for a psychiatrist".

Sanders and the pope met briefly on Saturday at the Vatican guest house where Francis lives and where Sanders spent the night after addressing a Vatican conference on social justice.

"When I came down, I greeted him, I shook his hand and nothing more. This is called good manners and it is not getting involved in politics," the pope told reporters in answer to a question aboard the plane returning from the Greek island of Lesbos, where he visited a refugee camp..

"If anyone thinks that greeting someone is getting involved in politics, I recommend that he look for a psychiatrist," he said, laughing.

Earlier, Sanders told NBC News that the meeting was "very wonderful."

"I am not a Catholic, but there is a radiance that comes from him," Sanders said.

They touched on the topics of climate change and foreign policy during the 6 A.M. chat at the pontiff's residence, held shortly before the pope departed for the Greek island of Lesbos to meet with refugees.

Pope Francis is "one of the great leaders in modern world history," Sanders told NBC.

Sanders travelled to the Vatican to address a 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.