ROME, Aug 31 (Reuters) - Pope Francis on Wednesday announced major changes to the Vatican bureaucracy, slimming down the number of offices and giving himself direct charge of migration issues, continuing a reform push he promised when elected more than three years ago.

In a document known as a Motu Proprio, Latin for "by his own initiative", the pope said he would merge four Vatican offices into a "Dicastery for the Service of Integral Human Development" starting on Jan. 1.

When elected in 2013, Francis pledged to cleanse the Church's bureaucracy, which had been rocked by scandals and charges of greed and corruption. He said he wanted "a poor Church" that served the poor.

The pope will oversee work on migration and refugees within the new dicastery, or department, which will absorb the offices for justice and peace, human and Christian development, immigration, and health workers.

It will spearhead the Church's humanitarian work internationally, including oversight of funds allocated to charities, focusing on "migrants, those in need, the sick, the excluded and marginalized, the imprisoned and the unemployed, as well as victims of armed conflict, natural disasters, and all forms of slavery and torture," the pope wrote.

Francis will "temporarily" take personal charge of migration because "there cannot be a service for integral human development without paying particular attention to the phenomenon of migration."

The leader of the world's 1.2 billion Roman Catholics, has often defended refugees and urged Catholic parishes in Europe to host them, with limited success.

In April, after visiting a migrant camp in Greece, the pope brought three families of Syrian refugees back to Rome with him.

(Reporting by Steve Scherer; Editing by Robin Pomeroy) (( ; +39-06-8522-4369;))