Mexico's president pledged to pursue the truth in the case of 43 students who disappeared last year, as frustrated families accused authorities of lying about what happened.

Enrique Pena Nieto spoke for almost three hours with parents of those missing in a Mexico City museum ahead of Saturday's first anniversary of a tragedy bedeviling his administration.

The mothers and fathers, who were almost half-way into a 43-hour fast in honor of their sons, presented eight demands to Pena Nieto in a document charging that the authorities had manufactured a "historic lie."

Presidential spokesman Eduardo Sanchez said Pena Nieto signed the document and ordered the attorney general's office, as well as the interior and foreign ministries, to analyze it.

"We are on the same side and we are working on the same goal: To know what happened to your sons and punish each and every one of those who are responsible. We are searching for the truth together," Pena Nieto told the parents at the closed-door meeting, according Sanchez.

"The president made it very clear that the investigation remains open, it was never closed, it will not be shelved," Sanchez said.

But Vidulfo Rosales, a lawyer for the parents, said the president had not yet "committed to fulfilling any" of the demands and that the promises made by Pena Nieto were not new.

After the meeting, only the second since the students went missing, the parents returned to the capital's historic Zocalo square, where they set up camp on Wednesday.

"We won't rest, we will be a pebble in his shoes. We won't go home," said one mother, Maria de Jesus Tlatempa.

- Investigation questioned -

Sanchez said Pena Nieto ordered the creation of a special prosecutor's office to investigate the disappearances, though the spokesman did not explain how it differed from a similar unit created in 2013.

The parents asked that the unit be placed under international supervision, but Sanchez said it was up to the foreign ministry to look into it.

The attorney general's office has come under criticism from the parents and human rights groups, which have accused it of prematurely declaring that the students were all slaughtered.

The students, from a rural teacher college in the southern state of Guerrero, disappeared after they were attacked by local police in the city of Iguala on September 26, 2014.

Prosecutors say police then delivered the young men to the Guerreros Unidos drug gang, which killed them and incinerated their bodies after confusing them with rivals.

But the official investigation was questioned earlier this month by independent experts from the Inter-American Commission on Human Rights (IACHR), who said there was no evidence that the students were burned in a funeral pyre at a garbage dump.

Only one student was positively identified among charred remains although the attorney general said last week that there was a possible DNA match for a second one.

The experts urged the government to open a new line of investigation into whether they were attacked because they inadvertently seized a bus used by the cartel to transport heroin.

Sanchez said Pena Nieto ordered prosecutors to incorporate the recommendations of the IACHR experts.

The spokesman reiterated that more than 100 people have been arrested, including several Iguala police officers and the mayor, who is accused of siccing the cops on the students.

- Protest on Saturday -

The families and other students will join a demonstration in Mexico City on Saturday to mark one year since the disappearance.

Pena Nieto, meanwhile, will travel to New York that night for the United Nations General Assembly, despite past criticism for taking trips at times of tension in the country.

Sanchez defended the trip, citing "the importance of Mexico's participation."