Former president Vicente Fox has urged the current Mexican government to make it clear that a Donald Trump presidency would be unacceptable to Mexico.

"You cannot work with a crazy guy," Fox said of the Republican presidential nominee.

In an interview at his ranch-turned-presidential library in the central Mexican state of Guanajuato, Fox added that reasons of "political correctness" were not enough to persuade him of the need to "accept as my neighbor country's president a crazy guy."

The former leader's comments directly challenged the recent softening of the current Mexican government's tone on Trump.

During a visit to the White House last month, President Enrique Peña Nieto denied he had ever compared the candidate to Hitler and Mussolini — something which he had done in March.

"I can't agree with some of things he has said, but I will be absolutely respectful and will seek to work with whomever becomes the next president of the United States," Peña Nieto underlined his new position in an interview on the Televisa network broadcast this Tuesday. The president also said he would be happy to meet with the candidate.

Fox, meanwhile, has made it clear he wants to remain one of the loudest critics of Trump, and his pledge to build "a beautiful wall" along the 2,000-mile long border between the US and Mexico, and to make Mexico pay for it.

VICE News interview with ex Mexican president Vicente Fox

In one interview in February, the 74-year-old Fox pronounced, "I'm not going to pay for that fucking wall."

The former president has also maintained a steady anti-Trump tirade on his Twitter account. This has ranged from highlighting Made in Mexico labels in Trump suits to announcing he had not invited the candidate to his wife's birthday celebration in Cancún.

Fox chose his words rather more carefully in this week's interview.

"We should be letting our voice be heard and demand respect to Mexico," he said. "Mexicans need to start debating those crazy ideas that he is after, using figures that do not correspond to truth."

Fox also compared Trump to charismatic left-wing Latin American leaders with whom he clashed during his six-year term that began in 2000.

He cited both Venezuela's President Hugo Chávez and Cuba's Fidel Castro, smiling as he recalled the time in 2002 when he told the Cuban leader "you eat and you go."

The Spanish phrase — comes y te vas, usually translated as eat up and go — comes from a phone call Fox made to Castro just before a summit in Mexico in which he tried to persuade the Cuban to leave immediately after lunch before the arrival of then US president George W. Bush. It came back to haunt Fox when Castro played a tape of the private conversation a few months later and accused him of being a US stooge.

But at the same time as he compared Trump to some of Latin America's most enduring leaders, Fox also appeared confident that the American will never become president himself. He celebrated the candidate's recent fall in the polls with a direct translation of a Mexican expression that warns against the possible consequences of being too loquacious. "I guess the fish is dying by the mouth," he said.