The United States and Mexico agreed on Friday to tighten security along their border and start sending illegal immigrants caught sneaking across the frontier back home by bus or plane.
US Homeland Security Secretary Tom Ridge and Mexican Interior Minister Santiago Creel signed an accord to step up co-operation on security on the border, seen by some as the soft underbelly for the US war on terror.
The agreement includes a controversial plan to start repatriating illegals to their hometowns rather than simply dumping them on the Mexican side of the frontier.
The two countries have yet to work out details of the repatriation plan, a touchy issue for Mexicans sensitive to any sign of US interference in their internal affairs.
"Together we need to reinforce secure and orderly repatriation of migrants to their places of origin," Ridge told a news conference in Mexico City.
Mexico is also keen to ramp up security on the 2,000-mile (3,200 km) border to cut the deaths of hundreds of Mexicans who perish every year making the dangerous illegal crossing in search of a higher standard of living in the United States.
Creel tried to calm fears that a forced repatriation inside Mexico would be against Mexican law.
"Our constitution guarantees free movement inside our territory and of course we are going to comply strictly and exactly with the constitution," Creel said.
Mexican President Vicente Fox's government was criticised for allowing US Transportation Security Administration agents to direct some security operations at Mexico City airport during increased vigilance of US-bound flights in January.
Ridge said there was no evidence of terrorists entering the United States from Mexico but warned that networks of drug smugglers and people traffickers in Mexico could be used by terrorists.