Nationalists, especially in military and intelligence circles, have long harbored conspiracy theories that foreigners are scheming to take Amazon resources.

But in recent months -- a period that has coincided with a spike in destruction of the world's largest forest -- they have become louder and more public.

Some legislators are concerned about foreign businesses buying land in the Amazon.

"The growing acquisition of land by foreigners in the Amazon is a threat to our national security, we need to impose restrictions now," Sen. Joao Pedro told Reuters.

The government accuses some non-governmental organizations of biopiracy -- stealing medicinal plants for pharmaceutical purposes -- but has provided little evidence.

The government said this month it could shut down foreign NGOs that fail to provide detailed accounts of their operations. They must register with half a dozen authorities, including the Federal Police, and reveal the qualifications and residence of their directors.

"We want to separate the wheat from the chaff," said Secretary of Justice Romeu Tuma Junior.

"The state has the right to demand that those wanting to operate in an area of national interest open their books publicly," said Tuma.

Conservationists say they are being scapegoated and worry about potential censorship.

"It's a first step toward ideological control of foreigners acting here," said Raul Telles do Valle of the social and environmental think tank ISA.

"These non-government organizations are sometimes a front for economic interests of hegemonic nations. They are predators and spies, seeking the biodiversity of our Amazon," retired Col. Manoel Soriano Neto, a former Army intelligence officer, told the government news agency.

DEFORESTATION RATE RISING

Critics say the growing opposition to foreign influence is a reaction to calls for more international intervention to combat deforestation.

Since August, the rate of deforestation has risen sharply after three years of declines. Figures released on Tuesday showed 423 square miles (1,096 square km) of forest were lost in May.

"There are cycles of xenophobia in Brazil; this one follows a new international spotlight on the Amazon," said Adrian Garda, Amazon program director with Conservation International.

"I've heard many accusations of biopiracy but seen no proof," said Garda, adding that his group won't follow the government's foreign NGO registration request because it considers itself Brazilian.

Brazil considers a large-scale foreign invasion of the Amazon as a possible, albeit remote, security threat, Defense Minister Nelson Jobim said in April.

In May, the Army chief for the Amazon warned that Brazil's borders were vulnerable to foreign incursions through tribal Indian territories, which harbored foreign aid workers.

President Luiz Inacio Lula da Silva said that foreign concerns over Amazon destruction from beef, soybean and ethanol production were attempts by rivals to undermine Brazilian exports.

Blaming foreigners may stem from frustration with Brazil's inability to control vast areas of its territory, some say.

"It's a scandal that foreigners can buy land in the Amazon without any state control," said Sen. Jose Nery of the northeastern Amapa state.

Critics say a nationalistic stance is good for politicians' poll numbers too.

"They're being populists, that wins votes," Valle said.

(Editing by Eric Beech)