Experts from a regional human rights body investigating the disappearance of 43 students in Mexico complained Monday that government delays in allowing them to interview soldiers could limit their work.

The Inter-American Commission on Human Rights has been asking since March to meet with soldiers in the city of Iguala, where the students were last seen in 2014.

"More than three months after having made the request to the authorities to directly interview soldiers of the 27th Battalion... the group of experts notes with concern the delay" in getting access, said a statement read by Colombian attorney and commission member Angela Buitrago.

Mexican prosecutors say corrupt local police officers in Iguala grabbed the students as they rode on stolen buses in September and handed them over to a drug gang, which slaughtered them and incinerated their bodies.

Parents of the students refuse to believe they are dead and have questioned whether soldiers had some kind of role in the disappearance or at least failed to act to stop the attack.

The government has denied that the army had any role in the case.

Commission expert and Chilean attorney Francisco Cox warned that the longer they wait to talk with soldiers, "everything becomes more complicated" and it "limits the possibilities for searches and possible investigations."

The experts say the government told them Sunday that it was still analyzing their request and that they could ask prosecutors to provide the statements given by soldiers to the authorities.

While interviewing the soldiers is "relevant" to the commission's investigation, Cox said that failing to speak with them would not compromise their final conclusions.

The commission is due to present a final report in September.