Belgium said on Monday it was investigating suspected foreign state espionage against its main telecoms company, which is the top carrier of voice traffic in Africa and the Middle East, and a newspaper pointed the finger at the United States.

Federal prosecutors said in a statement that the former state telecoms monopoly Belgacom had filed a complaint in July about the hacking of several servers and computers.

"The inquiry has shown that the hacking was only possible by an intruder with significant financial and logistic means," they said.

"This fact, combined with the technical complexity of the hacking and the scale on which it occurred, points towards international state-sponsored cyber espionage." The prosecutors declined to say which foreign state they suspected.

Documents leaked by the former US intelligence contractor Edward Snowden have revealed a vast National Security Agency (NSA) electronic surveillance program that has tracked enormous quantities of both Americans' and foreigners' email and telephone data.

This has mostly been done by agreement with telecommunications or internet providers or foreign government intelligence agencies.

However, the Brazilian television network Globo this month alleged that the NSA had tapped into the computer systems of companies including Google Inc and the Brazilian state oil firm Petroleo Brasileiro, as well as France's Foreign Ministry and the Belgian-based international banking cooperative SWIFT, which handles international financial transactions.

The Belgian daily De Standaard said the NSA had been monitoring international telephone traffic through Belgacom for two years. It said the hackers had been primarily interested in Belgacom's subsidiary BICS, a wholesale provider of international phone lines and biggest voice carrier in Africa and the Middle East.

State not named

The government of Belgium, which has a majority stake in Belgacom, condemned the intrusion and the violation of a public company.

Prosecutors said the intruder had used malicious software and advanced encryption techniques with the aim of gathering strategic information, and had not sabotaged Belgacom's data or sought to cause direct economic damage.

Belgacom itself said it had carried out work over the weekend to improve security and remove an unknown virus from its internal systems, adding that at the moment there was no indication of any impact on customers or their data. It said it had filed a complaint against an unknown third party.

Globo's revelations were based on leaked US documents provided by Guardian journalist Glenn Greenwald, who has worked with Snowden to expose the extent of US spying.

James Clapper, the US Director of National Intelligence, responded last week by saying US agencies did collect information about economic and financial matters, which was used to combat terrorist financing and predict problems that could lead to financial crises or disrupt financial markets.

"What we do not do," Clapper said in a statement, "is use our foreign intelligence capabilities to steal the trade secrets of foreign companies on behalf of - or give intelligence we collect to - US companies to enhance their international competitiveness or increase their bottom line."