Brazil and Mexico summoned US ambassadors on Monday to demand explanations over allegations that the National Security Agency spied on their presidents' communications.
Brazil's Foreign Minister Luis Figueiredo said the interception of Internet data from President Dilma Rousseff reported by US journalist Glenn Greenwald, if proven, "represents an unacceptable and unallowable violation of Brazilian sovereignty."
In Mexico, the foreign ministry said it sent a diplomatic note to Washington calling for an "exhaustive investigation" into claims that the NSA spied on President Enrique Pena Nieto and warned that, if true, the snooping would be a "violation of international rights."
The claims reported by Greenwald, who obtained secret files from NSA leaker Edward Snowden, follow allegations of widespread US electronic espionage in Latin America that angered the region's leaders.
The report emerged as Rousseff and Pena Nieto prepare to travel to Russia later this week for a Group of 20 summit during which they will see US President Barack Obama.
Rousseff is also scheduled to visit Washington in October.
Greenwald, an American who lives in Rio de Janeiro and writes for Britain's Guardian newspaper, told Globo television that the NSA tracked Rousseff's use of the Internet and accessed Pena Nieto's emails before his election last year.
Rousseff called a cabinet meeting and Figueiredo later said he told US Ambassador Thomas Shannon that his government wanted "a formal, written explanation ... as soon as possible, this week."
Justice Minister Jose Eduardo Cardozo said Brazil would wait for a response "and from there examine what measures to take."
"We are in an emergency situation due to these spying allegations," presidential secretary general Gilberto Carvalho was quoted as saying by O Estado de Sao Paulo newspaper.
The US embassy in Brasilia declined to comment on the meeting and a State Department spokeswoman in Washington said she was looking into the matter.
The Mexican government later said it had summoned US Ambassador Earl Anthony Wayne but there was no immediate word on when the meeting would take place.
The Brazilian Senate plans to name a special committee on Tuesday to investigate allegations of US spying in Brazil.
Senator Ricardo Ferraco, head of the Senate foreign relations committee, denounced the US government's "lack of limits."
Greenwald told Globo on Sunday that a document dated June 2012 shows that the NSA was trying to better understand Rousseff's methods of communication and interlocutors using a program to access all Internet content the president visited online.
The NSA program allegedly allows agents to access the entire communications network of the president and her staff, including telephone, Internet and social network exchanges.
Rousseff held a working meeting to study the revelations in the Globo report, the channel said.
The June 2012 document also said that Pena Nieto's emails were being accessed, one month before he was elected in July last year.
The NSA intercepted some of his voicemails, which included messages in which the future leader discussed the names of potential cabinet members.
In July, Greenwald co-wrote articles in O Globo newspaper revealing that the US government had a joint NSA-CIA base in Brazil to gather data on emails and calls flowing through the country.
The issue was discussed during a visit by US Secretary of State John Kerry in August, but Brazil said it was not satisfied by Washington's explanations.
Cardozo, the justice minister, met with US Vice President Joe Biden in Washington last week to discuss the matter and said the United States rejected a Brazilian offer to negotiate a bilateral agreement on surveillance.
Snowden, a former NSA contractor, is now a fugitive in Russia under temporary asylum.
He is wanted by Washington on espionage charges linked to media disclosures about US surveillance programs.