The U.N. Office of the High Commissioner for Human Rights said Tuesday it is troubled by a group of international experts' complaints of obstacles to their investigation into the disappearance of 43 students in Mexico.

Spokesman Rupert Colville said in a statement that the office is "concerned about the many challenges and obstacles reported by the experts," including the ability to examine other lines of investigation such as military and other officials' possible roles in the case.

He called on the Mexican government to "take into serious consideration" the recommendations of the group of experts from the Inter-American Commission on Human Rights.

The group's report from Sunday criticized the government's investigation of the 2014 disappearances. It said suspects were apparently tortured and key pieces of evidence were not investigated or handled properly.

Government investigators have said the students were taken by local police in the city of Iguala, in the southern state of Guerrero, and handed over to drug gang members who killed them and burned the bodies at a trash dump.

The group of experts, known by the acronym IGIE, and a separate body made up of Argentine investigators say there is no evidence at the dump of a fire large enough to incinerate that many corpses.

Mexican President Enrique Pena Nieto said Sunday via Twitter that the federal attorney general's office would "analyze the whole report, to aid in its investigations."

Colville called the group's work "invaluable" and urged the government to explore new lines of investigation.

"It is very important that the Government acts decisively on the IGIE's recommendations and ensures the rights to truth and justice of the victims and their families." Colville said.

The United States also called on Mexico to consider the experts' recommendations.

Parents of the missing students were planning a protest later Tuesday on the 19-month anniversary of the disappearances.

Late Monday, Mexican prosecutors announced the detention of a suspected member of a drug gang linked to the disappearances.