Ms Lopez Urbina will head a federal-level investigation

More than 250 women have been killed - a third of them also sexually assaulted - in Ciudad Juarez, close to the US border over the past decade.

Local police have been criticised by rights groups for being inefficient.

The newly-appointed Maria Lopez Urbina lifts the investigation to a federal level. She said claims of negligence by officials would also be looked into.

Chihuahua state police were slammed by Amnesty International in August as bungling and corrupt, accusing them of tampering with evidence and obtaining confessions through torture.

"I will investigate the cases where there is evidence of inefficiency, negligence or tolerance on the part of public servants so there is no more impunity for those who failed to fulfil their duty," she said.

In October, President Vicente Fox appointed a human rights lawyer Guadalupe Morfin to head a commission coordinating the efforts of agencies investigating the slayings.

She will work with Ms Lopez Urbina on investigating the Ciudad Juarez killings.

Cautious welcome

Rights activists gave a cautious welcome to the appointment.

"It won't make any difference unless she is given authority to take over all of the cases," said rights activist Oscar Maynez.

Many of the women killed in Ciudad Juarez - across the border from the US city of El Paso in Texas - over the past 10 years were factory workers snatched while travelling to and from their jobs.

Most had been brutally sexually assaulted and tortured before their deaths.

There have been several arrests - but most cases were allegedly based on forced confessions and only one man has been convicted, for one of the killings.

The murders first came to light in 1993, when bodies of dead girls were found in dusty desert graves and by roadsides in Ciudad Juarez.

It is not known whether the murders were committed by a serial killer or killers, or if criminal gangs are involved.