By Anthony Boadle

BRASILIA (Reuters) - Brazil assailed the United States on Monday after new allegations that Washington spied on President Dilma Rousseff, complaining that its sovereignty may have been violated and suggesting that it could call off Rousseff's planned state visit to the White House next month.

A Brazilian news program reported on Sunday that the U.S. National Security Agency spied on emails, phone calls and text messages of Rousseff and Mexican President Enrique Pena Nieto, a disclosure that could strain Washington's relations with Latin America's two biggest nations.

Mexico asked the United States to investigate the allegations, saying they would be a serious violation of its sovereignty if proven true.

Brazil's government, already smarting from earlier reports that the NSA spied on the emails and phone calls of Brazilians, called in U.S. ambassador Thomas Shannon and gave the U.S. government until the end of the week to provide a written explanation of the new spying disclosures based on documents leaked by fugitive former NSA contractor Edward Snowden.

"I expressed to (Shannon) the Brazilian government's indignation over the facts revealed in the documents," Foreign Minister Luiz Alberto Figueiredo said at a news conference.

"From our point of view, this is an inadmissible and unacceptable violation of Brazilian sovereignty," he said.

Figueiredo declined to explicitly say whether the allegations could lead Rousseff to call off her visit to Washington, the only state visit offered by President Barack Obama this year. The trip had been intended to highlight improving U.S.-Brazil ties since Rousseff took office in 2011.

But, in response to a question from reporters about the visit, he said that Brazil's response to the allegations "will depend" on the U.S. explanation.

The report by Globo's news program "Fantastico" was based on documents obtained from Snowden by journalist Glenn Greenwald, who lives in Rio de Janeiro and was listed as a co-contributor to the report.

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"Fantastico" showed what it said was an NSA slide dated June 2012 displaying passages of written messages sent by Pena Nieto, who was still a Mexican presidential candidate at that time. In the messages, Pena Nieto discussed who he was considering naming as his ministers once elected.

A separate slide displayed communication patterns between Rousseff and her top advisers, "Fantastico" said, although no specific written passages were included in the report.

Both sLides were part of an NSA case study showing how data could be "intelligently" filtered by the agency's secret internet surveillance programs that were disclosed in a trove of documents leaked by Snowden in June, "Fantastico" said.

STATE VISIT, F-18 JETS

The Brazilian Senate, where some members have proposed offering Snowden political asylum in Brazil, launched an inquiry into the secret surveillance of Brazilian Internet communications by the NSA.

Rousseff held a Cabinet meeting on Monday that included the country's defense, justice, communications and foreign ministers to discuss a response to the new espionage report.

The White said it would respond to the requests of its "partners and allies" through diplomatic channels.

"While we are not going to comment publicly on every specific alleged intelligence activity, as a matter of policy we have made clear that the United States gathers foreign intelligence of the type gathered by all nations," said White House spokeswoman Caitlin Hayden.

Mexico's Foreign Ministry said that although it could not comment on the veracity of the report, it "categorically rejects and condemns any kind of spying against Mexican citizens in breach of international law."

A ministry statement added that Mexico had asked the U.S. government for a thorough investigation of the matter and that, if necessary, Washington should explain who was responsible.

"Mexico's foreign ministry has summoned the U.S. ambassador to express its clear dismay and to demand that the aforementioned investigation be carried out," it said.

In July, after initial reports of NSA surveillance of internet communications in Latin American nations, Mexico's Pena Nieto said it would be "totally unacceptable" if it were revealed that the United States had spied on its neighbor and largest business partner in the region.

The United States is hoping to sell Brazil 36 F-18 fighter jets, but a Brazilian government official said manufacturer Boeing's chances of landing the more than $4 billion deal have been set back by the espionage scandal.

During a visit last month, U.S. Secretary of State John Kerry urged Brazil not to let spying revelations derail growing trade, diplomatic and cultural relations between the two largest economies in the Americas. But he gave no indication the United States would end the secret surveillance.

Kerry said the NSA surveillance was aimed at protecting Americans and Brazilians from terrorist attacks.

Justice Minister Jose Eduardo Cardozo said on Monday the latest revelations based on Snowden's documents show that U.S. electronic surveillance goes beyond combating terrorism and has political targets and may even involve commercial espionage.

Cardozo traveled to Washington last week and met with U.S. Vice President Joseph Biden and other officials, seeking more details on the previous, seemingly less serious set of disclosures by Snowden regarding U.S. spying in Brazil.

(Additional reporting by Dave Graham in Mexico City, Jeff Mason in Washington, Esteban Israel in Sao Paulo and Alonso Soto in Brasilia; Editing by Brian Winter and Mohammad Zargham)