By Steve Holland and Anthony Boadle
ST. PETERSBURG, Russia/BRASILIA (Reuters) - President Barack Obama promised on Friday to look into a report the United States spied on the leaders of Brazil and Mexico, allegations that have caused tensions in Washington's ties to its two biggest Latin American partners.
Obama met with presidents Dilma Rousseff of Brazil and Enrique Pena Nieto of Mexico during an international summit in Russia and discussed reports that the U.S. National Security Agency snooped on their personal communications and phone calls.
"I assured them that I take these allegations very seriously. I understand their concerns. I understand the concerns of the Mexican and Brazilian people; and that we will work with their teams to resolve what is a source of tension," Obama said at a news conference.
Rousseff, speaking earlier on Friday, indicated she was not fully satisfied with Obama's assurances during their meeting late on Thursday. She said Obama had agreed to provide a fuller explanation for the reported spying by Wednesday, and that she would decide whether or not to go ahead with a planned visit to the White House next month based in part on his response.
"My trip to Washington depends on the political conditions to be created by President Obama," Rousseff told reporters before leaving Russia.
Mexico's leader Pena Nieto said Obama made a personal pledge to investigate the alleged spying by the NSA to avoid the issue damaging relations with the U.S.'s largest trade partner in Latin America.
Brazil's TV Globo reported on Sunday that the NSA monitored the emails, text messages and phone calls of Rousseff as president and Pena Nieto when he was a candidate. The report was based on documents leaked by fugitive former NSA contractor Edward Snowden.
The report angered Rousseff and her government has demanded a fuller explanation, arguing that counterterrorism or cybersecurity concerns did not adequately explain why the NSA would spy on Rousseff's communications with her top aides.
Brazil has been a democratic U.S. ally in South America for decades, and is not a known base for terrorists.
Brasilia has already called off a trip by an advance team to prepare for next month's visit to Washington.
Obama said the tensions over the NSA spying disclosures should not override the wide-ranging relationship the United States has with Brazil, which he called "an incredibly important country" and an "amazing success story."
He said the NSA's job was to gather data not available through public sources, much as intelligence services from other nations do to track threats of terrorism, except that the U.S. agency's capacity to collect information was far bigger.
Obama acknowledged that the U.S. government needed to "step back and review what it is that we're doing" and do a cost-benefit analysis of how useful the information was given legitimate concerns around privacy and civil liberties.
"It's important for us, on the front end, to say, all right, are we actually going to get useful information here. And if not, if it's not that important, should we be more constrained in how we use certain technical capabilities," he said.
Rousseff is due to make a formal state visit to Washington on October 23 to meet U.S. President Barack Obama and discuss a possible $4 billion jet-fighter deal, cooperation on oil and biofuels technology, as well as other commercial agreements.
(Reporting by Steve Holland in St. Petersburg, Anthony Boadle in Brasilia and Liz Diaz in Mexico City; Editing by Brian Winter and Tim Dobbyn)