Pope Francis has appealed for unity and family love in an open-air Mass in Philadelphia wrapping up his six-day visit to the United States and Cuba.

"We renew our faith in the word of the Lord which invites faithful families to this openness," he said at the VIII World Meeting of Families.

"It invites all those who want to share the prophecy of the covenant of man and woman, which generates life and reveals God," said the Pope in his homily before thousands of people on the Benjamin Franklin Parkway.

Earlier, the pontiff prayed with a group of victims of child sexual abuse, saying "God weeps" for their suffering.

Clergy and bishops abusing children will be held accountable, he said in a country where the Roman Catholic Church has faced accusations of covering up child sex scandals in the past.

That has led to some U.S. dioceses having to make huge compensation payments to victims and accusations from survivors that Vatican has not dealt properly with child-abuse allegations.

As has become a custom for the Argentine pope, he gave a wide-ranging press briefing en route from the United States to Rome.

Pope Francis spoke on the thorny subject of forgiving priests who have molested minors, saying that the strength to forgive, and to be forgiven, can only come from God, Catholic News Agency reports.

Francis also told journalists that while he is willing to forgive clergy who have abused children and young people, he understands why there are some who find it difficult.

"We must forgive, because we were all forgiven," the Pope said. However, "it is another thing to receive that forgiveness."

The Catholic Church leader told journalists aboard the papal plane he is not judgmental of victims or the families of molested children who struggle to forgive the abuser.

He cited a meeting with a meeting once with a victim of molestation, who told him her mother had "lost her faith and died an atheist" due to the abuse.

"I understand that woman," the Pope said, "and God who is even better than me understands her."

Pope Francis said he believes this mother "has been received by God," taking into account that it was her own flesh and blood, her daughter, who was molested.

"I don't judge someone who can't forgive," he said, but said instead he prays for them. "God is a champion in finding paths of solutions. I ask him to fix it."