Central Americans who cross Mexico to illegally enter the US risk becoming victims of criminal gangs [EPA]

Armed men have kidnapped nine migrants in a southern Mexican state where 50 people disappeared last week.

El Salvador's foreign ministry said on Monday that five of the migrants escaped and reported the kidnapping. Another was killed as he tried to flee and the other three remain missing.

The ministry said in a statement, quoting the survivors, that seven men kidnapped the Central Americans on December 21 from a train near the Ixtepec, a town in Oaxaca state.

It said that Mexican authorities are investigating the incident and already have the five witnesses in protective custody.

Mexican police found the body of the dead migrant, identified as Tomas Ferman Pineda, 41, a Salvadoran national, and were arranging to repatriate his body.

The nationality of the other migrants was unclear.

Migrants 'kidnapped'

The incident comes a week after Mexican authorities announced that they were investigating the possible kidnapping of 50 illegal migrants in Oaxaca, after initially saying there was no evidence of the crime.

El Salvador's foreign ministry was also first to report the December 16 incident.

Witnesses say the migrants - 30 men, 15 women and five children - were held up by the armed men while trying to cross the country by train.

Honduran, Guatemalan and Salvadoran migrants have been interviewed by officials at Mexico's federal attorney general's office about the assault. But Oaxaca state authorities said on Monday that investigators have no leads despite scouring the train route in the region.

"We have nothing, there is no evidence of a kidnapping, and state and federal operations have not produced favourable results," Manuel de Jesus Lopez Lopez, Oaxaca state's attorney general, said.

Mexico, where drug gangs are staging a bloody turf war, is also the transit route for thousands of illegal migrants seeking to reach the US.

The government's National Human Rights Commission reported in 2009 that nearly 10,000 migrants are kidnapped a year by gangs.

In the most shocking case to date, 72 dead migrants were found in the northeastern state of Tamaulipas in August, a massacre blamed on members of the Zetas drug gang, which controls transport routes in that area for drugs and other contraband.