The Vatican will study the modern-day killing of Christians and try to list "all those who have shed their blood" in the name of Christ since the turn of the century, Pope Francis said on Wednesday.

In a letter released by the Vatican, Francis said he had given the task to the "Commission for New Martyrs - Witnesses of the Faith", a new working group within the Vatican's saint-making department.

The move, linked to the upcoming 2025 Jubilee year, follows on from a similar initiative taken by the late Pope John Paul II for the 2000 Jubilee, and comes on the back of renewed concerns about the persecution of Christians around the world.  

"Martyrs are more numerous in our time than in the first centuries: they are bishops, priests, consecrated men and women, lay people and families, who in the different countries of the world, with the gift of their lives, have offered the supreme proof of charity," Francis said.

In a gesture of outreach towards other Christian churches, the pope said the new committee would seek to document killings involving "all Christian confessions", rather than just Catholics.

Francis renewed his assertion that an "ecumenism of blood" was uniting persecuted Christians of all denominations, and said they would be commemorated with an ecumenical service during the 2025 Jubilee - as had been done in 2000.

In May, the pope said 21 Coptic Orthodox Christians beheaded by Islamic State in Libya in 2015 had been recognised as Catholic martyrs, and called for freedom of worship in China, where religious minorities, including Catholics, have long faced harassment.