Brazil's President Dilma Rousseff reacts during a reception for the Brazilian Football delegation at the Alvorada Palace in Brasilia September 2, 2013. [Photo/Agencies]

Brasilia -- Brazil said Monday it considered claims the US spied on President Dilma Rousseff to be "very serious," and, if proved, were "an inadmissible and unacceptable violation of sovereignty."

Justice Minister Jose Eduardo Cardozo and Foreign Minister Luiz Alberto Figueiredo told a press conference after an emergency meeting with Rousseff they expected a formal written response from the US government.

According to the two ministers, the Brazilian government had demanded a prompt official explanation from US authorities because previous explanations were insufficient and unsatisfactory. The ministers said they had met US ambassador Thomas Shannon and set Friday as a deadline.

Figueiredo said Brazil's next steps would depend on the answers the US provided. Cardozo said US spying on a Brazilian head of state went against ties between two nations bound by a strategic alliance.

The ministers said Brazil was determined to take the matter to various international forums.

"We will be talking with our partners, both developed nations and emerging ones, such as those in BRICS, because we believe this involves all of us, affects all of us," Figueiredo said.

Asked whether Rousseff would go ahead with her scheduled trip to Washington next month, Figueiredo declined to reply. But local press reported the trip could be postponed or even cancelled.

Figueiredo said Brazil would promote a debate on establishing international regulations that prevent spying on private and government communications.

"We want an agreement to regulate Internet use, not to curtail rights, but to protect rights. We want there to be real freedom, so companies, citizens and governments won't be a target of these types of violations," Figueiredo said.

Rousseff has made no formal statements so far, but called the emergency meeting early Monday with several ministers to discuss the scandal.

The moves come after a Brazilian TV program claimed last Sunday US spying activities previously revealed by former US intelligence contractor Edward Snowden had reached as high as the Brazilian presidency. The report was based on documents Snowden had given Guardian reporter Glenn Greenwald, who lives in Rio and contributed to the Globo report. The program said the US National Security Agency intercepted the Brazilian president's phone calls and emails to aides.

In July, after news of US spying on Brazilian government agencies, companies and individuals first surfaced, Globo claimed Brazil was the NSA's largest target.

Both the frequency and content of communications were reportedly monitored, angering officials, who have condemned what they consider to be grave violations of Brazil's sovereignty and basic human rights.

Last week, Cardozo traveled to Washington, where he discussed the earlier spying revelations with US Vice President Joe Biden.

According to Sunday's Globo report, Mexican President Enrique Pena Nieto was also a target, with his emails being read before his election. The report also indicated Pena Nieto's voicemails were intercepted, including discussions on potential cabinet members ahead of his 2012 victory.

The Mexican Foreign Ministry said it had asked for an "exhaustive investigation" into the snooping and an official explanation from the US government, adding, if the claims were true, the US would have contravened international rights.