Pope Francis led a special prayer for peace in South Sudan and the Democratic Republic of Congo on Thursday, saying the mass murder of women and children is where "war shows its most horrid face".

"This evening, in prayer, we want to sow seeds of peace in the lands of South Sudan and the Democratic Republic of Congo, and in all lands devastated by war," the pope said in a homily at St. Peter's Basilica.

Referring to "walls of hostility" in the two countries, he lamented conflicts in which children have no part, but "which rob them of their childhood and at times of life itself".

"How hypocritical it is to deny the mass murder of women and children," he said. "Here war shows its most horrid face."

Earlier in the year the pope repeatedly said that he wanted to visit South Sudan with Archbishop of Canterbury Justin Welby, his Anglican counterpart but the Vatican announced in May that such a trip would be too dangerous.

South Sudan plunged into civil war in December 2013, two and a half years after it gained independence from Sudan. The conflict has left tens of thousands dead and forced a third of the country's 12 million people out of their homes.

The pontiff has also said he wished to visit DR Congo, but would not do so before elections are held there.

Violence flared in the vast country after President Joseph Kabila refused to step down as constitutionally mandated in December 2016.

Kabila has been in power since his father was assassinated in 2001. Elections are now planned for December 2018.

The United Nations said on Thursday that it had documented 704 human rights violations across DR Congo in October, including extra-judicial killings and rape, with children among the victims.