The United Nations in Mexico urged authorities Friday to conduct an "effective" search for 43 students who vanished after a deadly police shooting last week as fears mounted about their fate.
The UN statement came as relatives of the aspiring teachers met with interior ministry officials to plead for a "serious" federal investigation to find the students in the gang-plagued southern state of Guerrero.
Witnesses say they saw municipal police officers haul away the students in patrol cars after shooting at buses that the youngsters had hijacked in the town of Iguala on September 26.
Three students were killed in the shooting. Three other people died in a gun attack on a soccer team bus on the outskirts of town. Authorities have detained 22 officers over the shootings.
"The extreme gravity of the incidents linked to the disappearance of so many people make it one of the most terrible events of recent times," the UN in Mexico said.
"It compels us to make a humanitarian plea for those who have the missing to respect the life and integrity of the youngsters, to free them immediately, or give their location," it said.
The statement also called on the government to coordinate and reinforce "all measures at its disposal to conduct an effective and diligent search."
The UN's human rights body has dispatched personnel to the region and has contacted victims, families and local authorities.
Meanwhile, relatives met with Deputy Interior Minister Luis Enrique Miranda in Mexico City to demand a "serious search" by federal authorities.
"We believe that there's no serious, exhaustive, objective, responsible search preceded by a serious strategy that ensures success," said Vidulfo Rosales, a human rights lawyer representing the families.
Rosales said Miranda pledged to help with the search and that various federal agencies would meet to draw up a new strategy.
Relatives voiced concerns that municipal police officers handed the students to a violent drug gang. A gang is suspected in participating in the second shooting.
Scores of soldiers accompanied nine relatives who went door to door on Wednesday around Iguala, handing out pictures of the missing and asking for any information.
But relatives later complained that the search lacked any prior investigation by the authorities to figure out exactly where to search.