"The pope is profoundly saddened by their death," the Pope's spokesperson said.

Pope Francis expressed great sorrow at seeing a photo of a Salvadoran man and his 23-month-old daughter lying dead, face down in the Rio Grande river after trying to cross into the United States, the Vatican said on Wednesday.

The picture of Oscar Alberto Martinez Ramirez and his daughter Valeria has gone viral on social media in the United States and sparked renewed debate there on the plight of refugees and migrants from Central America.

"With immense sadness, the Holy Father has seen the images of the father and his baby daughter who drowned in the Rio Grande River," Vatican spokesman Alessandro Gisotti said.

"The pope is profoundly saddened by their death, and is praying for them and for all migrants who have lost their lives while seeking to flee war and misery."

The New York Times called the picture a "portrait of desperation" and said it could galvanize attention similar to the picture of refugee child Alan Kurdi who drowned in the Mediterranean and whose body washed up on a beach in Turkey in 2015.

The Vatican newspaper, L'Osservatore Romano, put the picture on its front page.

Francis, who was born in Argentina after his family emigrated there from Italy, has made defence of migrants and refugees a plank of his pontificate and has often clashed with U.S. President Donald Trump on immigration policy.