US Treasury Secretary John Snow has defended a secret programme which has been tracking international money transactions for nearly five years.
Tens of thousands of transactions were scrutinised
"This programme is an effective weapon in the larger war on terror," he said.
The scheme, which has sifted huge amounts of data from an international banking consortium, was revealed by the New York Times newspaper on Friday.
The US treasury says the programme was strictly confined to the records of suspected foreign terrorists.
Although there is no direct connection, the programme has echoes of a recently revealed US surveillance programme in which millions of international and domestic phone calls and e-mails were monitored, correspondents say.
They say that although the US government insists it acted on a firm legal footing, this programme is likely to elicit similar charges of enfringement of civil liberties.
"This programme is making a real difference," Mr Snow said. "It works. It's based on appropriate legal authorities, it has built-in safeguards and controls."
He said the treasury was using "the tools that Congress has given us to follow the flow of terrorist money".
"These flows ... lead to the terrorists themselves," he said.
The financial tracking scheme began in the aftermath of the 11 September 2001 attacks on the US using emergency powers invoked by President George W Bush, the New York Times revealed.
The newspaper spoke to some 20 anonymous current or former government officials or industry executives.
The government used powers of administrative subpoena to compel the Brussels-based banking co-operative, Swift, to open its records.
Swift - the Society for Worldwide Interbank Financial Telecommunication - links about 7,800 financial institutions around the world, including virtually every major bank and brokerage.
It provides banks with instructions on how to transfer funds - instructions which include information on the individuals sending money and the amount of money sent.
The treasury department said it had carried out "tens or hundreds" of thousands of name searches on the data.
Mr Snow said the programme had been implemented in a "highly responsible" manner, with corporate and external auditors observing the process.
In a statement on its website, Swift said: "Swift takes its role as a key infrastructure of the international financial system very seriously and co-operates with authorities to prevent illegal uses of the international financial system. Where required, Swift has to comply with valid subpoenas...
Messenger on fire
Officials quoted by the newspaper said the programme had helped in the capture of Hambali, blamed for a string of bombings across South East Asia.
But some expressed reservations, suggesting it was inappropriate for an urgent, apparently temporary programme to remain in operation for years without formal or congressional authorisation.
Others familiar with the programme told the paper they believed it exploited a "grey area" in the law.
Meanwhile, the New York Times came under fire from the White House for revealing the programme.
"We are disappointed that once again the New York Times has chosen to expose a classified programme that is working to protect Americans," said spokeswoman Dana Perino.
But the newspaper's executive editor Bill Keller said: "We remain convinced that the administration's extraordinary access to this vast repository of international financial data, however carefully targeted use of it may be, is a matter of public interest."