Pope Francis arrives to celebrate mass, in St. Peter's square at the Vatican. File picture: Andrew Medichini/AP

Vatican City - For Christians, helping the poor should be as much of a priority as fighting against abortion, Pope Francis said in a new papal document released Monday.

The Gaudete and Exsultate (Rejoice and be Glad) Apostolic Exhortation is a 44-page guide book from the leader of the world's 1.3 billion Catholics on how to be holy in the modern world.

"Our defence of the innocent unborn, for example, needs to be clear, firm and passionate, for at stake is the dignity of a human life, which is always sacred," Francis said.

"Equally sacred, however, are the lives of the poor, those already born, the destitute, the abandoned and the underprivileged, the vulnerable infirm and elderly exposed to covert euthanasia, the victims of human trafficking, new forms of slavery, and every form of rejection," he added.

Pope Francis said it was not permissible to see the plight of migrants as a "lesser issue" than "grave bioethical questions" such as the battle against euthanasia or abortion.

Francis is under fire from conservatives for focusing more on outreach efforts towards sinners and non-believers than on the enforcement of Catholic doctrine.

Worshipping, praying and "following certain ethical norms" is not enough to be a good Christian, because "the ultimate criterion on which our lives will be judged is what we have done for others," he warned.

Francis' last Apostolic Exhortation, 2016's Amoris Laetitia, included a softening of a long-standing ban on Communion for remarried divorcees that is still deeply controversial within the Catholic world.