Mexico ordered an investigation Tuesday into allegations that the United States spied on the emails of its president and his predecessor, the latest diplomatic wrangle stemming from America's intelligence-gathering efforts.
The row, following leaks from the former US intelligence contractor Edward Snowden, came after it emerged that millions of phone calls in France were reportedly being monitored by American government agencies.
The latter allegation led France to tell the United States to stop snooping on its citizens.
The anger in Mexico City and Paris has put Washington in an awkward position, given that the allies collaborate on everything from terror investigations to the war on drugs.
Mexican President Enrique Pena Nieto ordered an "exhaustive" probe into claims that the National Security Agency, headquartered close to the US capital, hacked his emails while he was running for office last year
Pena Nieto also alleged that former president Felipe Calderon had been subjected to US eavesdropping while in office.
The Mexican investigation will look into whether such spying indeed took place and if any local officials were complicit, said Interior Minister Miguel Angel Osorio Chong.
Since Pena Nieto took office in December, Mexico has "revised and strengthened" the security of the president's voice and computer communications, he added.
The allegations that Calderon was spied on from May 2010 were reported by German weekly Der Spiegel on Sunday after a similar report by US journalist Glenn Greenwald last month that Pena Nieto had been targeted by the NSA.
The Mexican leader has said that US President Barack Obama pledged to launch an investigation into the alleged spying on his emails.
But Foreign Minister Jose Antonio Meade said he would summon the US ambassador over the latest claims and that the American response to the spiraling scandal so far had been "unacceptable."
Calderon has described the news as an "affront to the institutions of the country."
In Paris, Foreign Minister Laurent Fabius used a breakfast meeting with US Secretary of State John Kerry to demand a full explanation of the latest revelations on the NSA's spying program.
"I said again to John Kerry what (French President) Francois Hollande told Barack Obama, that this kind of spying conducted on a large scale by the Americans on its allies is something that is unacceptable," Fabius said.
All the signs, however, were that France wants to defuse the row.
When asked if her country was considering reprisals over the NSA's conduct, government minister and spokeswoman Najat Vallaud-Belkacem replied: "I don't think there is any need for an escalation.
"We have to have a respectful relationship between partners, between allies. Our confidence in that has been hit but it is after all a very close, individual relationship that we have," she said.
Le Monde newspaper reported this week that the NSA had monitored more than 70 million phone communications in France between December 10, 2012 and January 8 this year.
The daily said the operation appeared to have targeted business and political figures as well as people suspected of being involved in terrorism, putting it in a different league from the monitoring that France's own intelligence services carry out.
Le Monde followed up on Tuesday by publishing details of US spying on French embassies around the world.
Known about for a very long time
Most commentators seemed to agree that it was far from surprising that the United States was conducting intelligence operations in France.
"It doesn't mean it is not very serious, and the Americans must explain themselves, but this has all been known about for a very long time," said Christian Jacob, an opposition MP.
"They're panicking and attempting a diversion operation."
Hollande told Obama on Monday evening that the NSA's actions had been "unacceptable between friends and allies."
Obama has already initiated a review of how America gathers intelligence with a view to addressing concerns over citizens' right to privacy, officials say.
US officials have argued that intelligence acquired from phone monitoring can benefit all of Washington's allies in fighting terrorism.
"Our goal is always to try to find the right balance between protecting the privacy and security of our citizens," Kerry said Monday.
The latest revelations come on top of previous disclosures by Snowden, who has taken refuge in Russia, that the United States had a vast secret program called PRISM to monitor Internet users.