Mexico's health minister has denied allegations by Fidel Castro that the country intentionally failed to reveal the existence of swine flu last month, BBC reported.
The former Cuban leader accused Mexico of covering up the outbreak in order to avoid missing out on a visit by US President Barack Obama.
The minister, Jose Angel Cordova, said Mexico had been extremely forthcoming and proactive over the outbreak.
It has been doing all it can to combat the H1N1 virus ever since, he added.
Mr Castro's comments came after Cuba reported its first confirmed case of swine flu on Monday. Correspondents say his remarks may contribute to a further worsening of relations between the two countries.
Cuba cancelled all flights to Mexico when news of the outbreak first emerged.
Mexico has confirmed the deaths of 58 people from swine flu and the spread of the virus caused a national crisis, with restrictions on public gatherings only relaxed last week.
Mexican President Felipe Calderon said last week he might cancel a planned trip to Cuba "as one of the unforeseen consequences of decisions that have no technical basis" - a clear reference to Cuba's ban on flights.
Mr Castro responded late on Monday in an internet column in which he referred to President Obama's visit to Mexico in mid-April, which came days before a spreading flu outbreak in the country was officially diagnosed as swine flu.
"The Mexican authorities did not inform the world of [the outbreak], awaiting the visit of Obama," the former Cuban leader wrote.
"Now they threaten us with suspending Calderon's trip."
A Mexican was diagnosed with swine flu in Mexico on Monday. He had become ill after returning from a trip home in late April.
"The only thing that can be confirmed now is that [the flu] wasn't brought here by the CIA," Mr Castro added in his column.
"It came from Mexico."