Criminal gangs, but also authorities, kidnapped almost 10,000 undocumented migrants in six months in Mexico, in an alarming, growing trend, according to the national human rights commission.

"The kidnapping of migrants has become a constant practice, on a worrying scale, generally unpunished and with characteristics of extreme cruelty," said the commission's head, José Luis Soberanes, on presenting the report.

Most of the 9,758 abducted between September last year and last February were from Central America, including almost 70 per cent from Honduras, and the average ransom demand was $2,500 per person, the report said.

Fifty-nine minors and 157 women were among those abducted, including four pregnant women, of which two were killed, it said.

Many were also raped, the report said, underlining the lack of official figures on the issue and slamming the "inefficiency" of the justice system in preventing and investigating the cases.

Most abductions were carried out by criminal groups, but Mexican authorities participated in the abductions of at least 91 migrants, the report said.

Some 500,000 mostly Central American undocumented immigrants try to enter Mexico each year across its southern border, according to the commission.