US President Barack Obama has promised to investigate allegations that the National Security Agency (NSA) spied on his Mexican and Brazilian counterparts, Mexico's leader has told the BBC.
President Enrique Pena Nieto said if the claims were true, Mr Obama had pledged to impose suitable penalties.
The allegations were made by a journalist who obtained secret files from fugitive US leaker Edward Snowden.
Brazil said if proven they were an unacceptable violation of sovereignty.
Mr Pena Nieto spoke to President Obama on the sidelines of the G20 summit in Russia, four days after Rio-based journalist Glenn Greenwald told a Brazilian TV station about the data interception allegations.
Mr Greenwald, a columnist for the British Guardian newspaper, alleged the NSA had monitored Mr Pena Nieto's communications even before he was elected in July last year.
The secret documents showed how US agents had also spied on communications between aides of Brazil's President Dilma Rousseff, he said.
The NSA had also used a program to access all internet content that Ms Rousseff visited online, he added.
"What I got from President Obama was a commitment to a full investigation... and if they turn out to be true to impose corresponding sanctions," Mr Pena Nieto told the BBC.
"I think this proves a commitment to preserve the cordial relations and the ever-widening horizon we have between our countries," he added.
Mr Obama also spoke to President Rousseff, who is scheduled to make her first state visit to the US in October, before dinner in St Petersburg.
Correspondents say there are now fears the row could jeopardise her trip.
Brazil's state-owned news agency Agencia Brasil has reported that a trip to Washington this coming Saturday by a delegation preparing for her visit has been cancelled.
They had been due to stay for five days, sorting out the schedule and facilities, it said.
US national security spokesman Ben Rhodes said the White House had not been informed of any delay in the planning of Ms Rousseff's visit, AFP news agency reports.
Earlier he told reporters: "What we're focused on is making sure the Brazilians understand exactly what the nature of our intelligence effort is.
"We carry out intelligence like just about every other country around the world. If there are concerns that we can address consistent with our national security requirements, we will aim to do so through our bilateral relationship."
Mr Greenwald was the first journalist to reveal the secret documents leaked by Mr Snowden on 6 June. Since then, he has written a series of stories about surveillance by US and UK authorities.