Mexican authorities have detained hundreds of undocumented migrants, mostly Central Americans, trying to make a transit across Mexico to reach the United States.

Media on Tuesday quoted officials as saying that more than 500 migrants as well as nine members of criminal gangs had been detained in six different operations over the weekend.

Some migrants were found crammed into “safe houses” designated by human traffickers while others were found in “un-ventilated trucks with no food or water.” Many were minors.

Human traffickers receive between $4,000 and $7,000 per person to smuggle people into the US.

Mexico’s criminal gangs are said to have secret tunnels running right across the border.

The operations came a week after Mexico detained more than 300 Central American men, women, and children being smuggled into the US.

US President Donald Trump is to grant citizenship to 1.8 million immigrants brought to the United States as children — a concession to opposition Democrats — in exchange for tough cutbacks on immigration and $25 billion for tighter border security, including his controversial pledge to build a wall on the US-Mexico border.

Amnesty International recently reported that Mexico deported thousands of Central Americans without taking into account their potential rights to treatment as refugees, such as when they are persecuted by brutally violent gangs in their home countries.

Most undocumented migrants who take the Mexican route to the US are Guatemalans and Hondurans.