Mexico warned Israel Friday that a lack of progress in efforts to extradite a former investigator in connection with one of the country's worst human rights tragedies threatened diplomatic relations.

Mexico has repeatedly asked Israel to hand over the ex-head of the Criminal Investigation Agency, Tomas Zeron, over allegations of serious irregularities in a probe into the disappearance of 43 students in 2014.

The two countries do not have an extradition treaty.

Zeron denies allegations of kidnapping, torturing suspects, manipulating evidence, and embezzling public funds.

"The lack of progress in resolving this case is interpreted as de facto protection by the Israeli government of Tomas Zeron and threatens to become an irritating and disruptive factor with the State of Israel," the Mexican foreign ministry said in a statement.

Zeron is one of the architects of the so-called "historical truth," the official version of the case presented in January 2015 by the government of the time that was rejected by the victims' families.

The students had been traveling to a demonstration in Mexico City when investigators believed they were kidnapped by a drug cartel in collusion with corrupt police.

The exact circumstances of their disappearance are still unknown, but a truth commission set up by the government has branded the case a "state crime," saying the military shared responsibility, either directly or through negligence.