A new study by human rights and migrants' advocates groups finds that Mexico detained 73 percent more migrants since the announcement of an operation to shore up its southern border region.

It says about 168,000 migrants were detained in Mexico from July 2014 to June of this year, up from some 97,000 during the previous 12-month period.

Mexico's Southern Border Program was announced in the summer of 2014, as the United States was experiencing a large spike in unaccompanied child migrants arriving at its own southern border. Many were Central American children fleeing violence in their home countries.

The Washington Office on Latin America helped prepare the report. It says the increased migration enforcement has "worrying implications" due to reports of migrants injured or killed during operations.