File photo - US Homeland Security Secretary Janet Napolitano.

The United States will increase repatriating undocumented Mexicans by air to take pressure off authorities in busy border crossing areas, visiting US Homeland Security Secretary Janet Napolitano said on Monday.

Napolitano made the announcement alongside Mexico's Interior Minister Alejandro Poire.

She said the change would ease pressure in areas along the countries' long border where many undocumented people from Mexico and other countries try to enter the United States illegally by land, at the same time that US authorities are carrying our deportations by land.

Napolitano did not give a time frame for the change.

Napolitano, speaking at at the start of a tour that will also take her to several Central American countries, said that Mexico and the United States also agreed to bolster anti-organised crime cooperation in their fight against the illegal drug trade which has shaken Mexico's legal system and society in recent years.

About 50 000 Mexicans have been killed in drug-related violence since the 2006 launch of a military crackdown on the cartels, which are engaged in brutal turf wars over smuggling routes and key ports.

At her next stop in Guatemala City, Napolitano shrugged off Guatemalan President Otto Perez' proposal to legalise drugs, saying the United States believed there were better ways to fight drug trafficking.

Perez believes that, to be effective, the legalisation plan should be adopted in all countries that are used as transit points as well as for production and consumption of illegal narcotics.

According to US data, 90 percent of cocaine shipped from South America to the lucrative US market passes through Central American nations like Guatemala. - Sapa-AFP