Pope Francis has expressed concern over the rise of nationalism in the world, saying leaders are weakening the system of global multilateral diplomacy.

In his annual address to ambassadors to the Holy See, the pope lamented national policies favoring 'quick partisan consensus' over committing to working towards long-term goals, but did not cite specific countries or political parties.

He said today's rise to power of populist and nationalist leaders mirrors what happened between the two world wars, when international efforts failed to prevent conflict.

Warning: Pope Francis said today's rise to power of populist and nationalist leaders mirrors what happened between World War I and World War II

'The reappearance of these impulses today is progressively weakening the multilateral system, resulting in a general lack of trust, a crisis of credibility in international political life, and a gradual marginalisation of the most vulnerable members of the family of nations,' he said.

He also said he had noted an increased worry in Europe and North America about migrants, and efforts to limit their entrance across borders.

The pontiff said 'partial solutions can't be given to such a universal question.'

He praised Colombia for accepting a large number of people fleeing Venezuela's economic difficulties.

The pope also voiced dismay over long-running conflicts in Ukraine and in Syria, and praised dialogue between North and South Korea, but warned against an accelerating global arms race.

Pope Francis attends an audience with the members of the Diplomatic Corps accredited to the Holy See for the traditional exchange of New Year greetings at the Sala Regia, at the Vatican

Pope Francis poses for a group photograph with members of the Diplomatic Corps accredited to the Holy See, inside the Sistine Chapel, at the Vatican

Pope Francis also addressed issues within the Catholic Church, specifically clergy sex abuse and covers-ups.

In his speech to the diplomats, Francis called the abuse of minors 'one of the vilest and most heinous crimes conceivable.'

He said the church was working to combat and prevent abuse and its concealment, to uncover church hierarchy's involvement and to deliver justice to minors who have 'suffered sexual violence aggravated by the abuse of power and conscience.'

The Catholic Church's credibility has been eroded by sex abuse by clergy and bishops and its often systematic concealment.

Francis called February's meeting 'a further step in the Church's efforts to shed full light on the facts.'

His own handling of some cases has drawn criticism.