By Philip Pullella

VATICAN CITY Pope Francis appealed to countries on Thursday to manage waves of migrants as best they can and to expand channels for their safe and regular movement, as the United States grapples with a surge of new arrivals at its southern border.

Francis made his comments in his message for the Roman Catholic Church's annual World Day of Migrants and Refugees, whose title this year is "Free to Choose Whether to Migrate or to Stay".

He called for a "shared commitment" to manage migration, with politicians in countries of origin implementing "transparent, honest and farsighted" policies and rich countries shunning any form of "economic colonialism" that exploits the natural resources of poorer countries.

"Persecutions, wars, atmospheric phenomena and dire poverty are among the most visible causes of forced migrations today. Migrants flee because of poverty, fear or desperation," Francis said, calling on countries to work together to eliminate the causes.

Francis, who has made defence of migrants and refugees a major part of his 10-year-old pontificate, said the aim of international cooperation should be to establish the right not to be forced to emigrate. He did not mention any countries.

"Even as we work to ensure that in every case migration is the fruit of a free decision, we are called to show maximum respect for the dignity of each migrant; this entails accompanying and managing waves of migration as best we can, constructing bridges and not walls, expanding channels for a safe and regular migration," he wrote.

Migrants have been massing in Mexico near various parts of the border with the United States - many of them unsure about when, or how, to cross. Drone footage showed large crowds gathering at the border fence by El Paso, Texas, across from Ciudad Juarez, Mexico.

Thousands are crossing before a new regulation takes effect that could bar most who cross illegally from seeking asylum in the United States.

(Reporting by Philip Pullella; Editing by Gareth Jones)