It is revealed as part of a Telegraph investigation which also unearthed an audio recording appearing to capture Litvinenko giving a detailed account of his investigations into links between Vladimir Putin and one of the world's most dangerous criminals.
The tape will reignite claims that Litvinenko could have been killed as a result of investigative work he carried out in a series of European countries after leaving Russia.
These claims are likely to be played out in the High Court as the Litvinenko Inquiry, chaired by Sir Robert Owen, a former high court judge, conducts its hearings.
Last year Sir Robert said that he had seen "prima facie" evidence that the Russia state was involved in the murder.
It is likely that the NSA intelligence formed part of the evidence that Sir Robert was given.
The disclosure of the material is likely to be put pressure on the British government's relationship with the Kremlin and will renew calls for the UK to toughen its stance.
The start of the inquiry comes after years of campaigning by Marina Litvinenko, the widow of the former KGB spy, for an official verdict on his death.
Mrs Litvinenko has applied to the NSA to disclose telephone intercepts, and says that "all information" should be disclosed to Sir Robert.
Litvinenko was poisoned in November 2006 during a meeting at a Mayfair hotel. He died three weeks later. Tests revealed that he ingested a rare isotope, polonium 210, which is hard to detect.
British prosecutors want two men, Andrei Lugovoy and Dimitri Kovtun, both of whom are former KGB bodyguards, to face murder charges over the murder.
Mr Lugovoy, now a Russian MP, and Mr Kovton, have always maintained their innocence and Moscow have said that they cannot be extradited under Russian law.
An international warrant has been issued for their arrest if they ever leave Russia.
Last October Marina Litvinenko filed a Freedom of Information request to the NSA through an intermediary asking for "NSA intercepts of telephone communications of Mr Andrei Lugovoy and Mr Dimitry Kovtun from London, UK, in the period October 15 to November 1 2006."
The application stated that the material was "to be used as evidence in the [public] inquiry hearings."
Paul Blaskowski, a senior NSA official, responded in a letter that it could not comment on the "existence or non-existence" of the transcripts because such material had "to be kept secret in the interest of national defence or foreign relations."
He said the spy agency was also empowered "to protect certain information concerning its activities" by withholding if from public disclosure.
Joel Brenner, who was Inspector General of NSA at the time of Litvinenko's murder, said that the "co-operation between the UK and US government on signals intelligence is extremely close and probably without parallel".
The public inquiry was ordered by Theresa May, the UK Home Secretary, last year.
It replaces an ongoing inquest and one of its key purposes will be to examine whether the Russian state was behind the killing.
The government had previously resisted calls for an inquiry, citing the need in part to "protect international relations."
But Sir Robert, who had also acted as coroner at the inquest, said that he could not conduct a "fair and fearless" investigation because the government had refused to release certain information.
This is thought to be intelligence material relating to the involvement of Russia in the case and of Litvinenko's work as an informant to MI6.
Sir Robert said last July that "sensitive" government evidence over the poisoning would now heard in closed sessions of the public inquiry.
The possible involvement of the Russian government in the murder would be of "central importance to my investigation", he said.
According to information that came out during the inquest, Litvinenko had been working for MI6 for several years during his time in London.
As part of this, Litvinenko also began assisting the Spanish security services. It is understood that his work in Spain involved investigating organised crime networks.
Litvinenko's work in Spain, as well as in Italy and Georgia, after leaving Russia and the KGB, has given rise to competing theories about who might have been behind his death.
Joel Brenner, who was Inspector General of NSA at the time of Litvinenko's murder, said that the "co-operation between the UK and US government on signals intelligence is extremely close and probably without parallel".
The public inquiry was ordered by Theresa May, the UK Home Secretary, last year.
It replaces an ongoing inquest and one of its key purposes will be to examine whether the Russian state was behind the killing.
The government had previously resisted calls for an inquiry, citing the need in part to "protect international relations."
But Sir Robert, who had also acted as coroner at the inquest, said that he could not conduct a "fair and fearless" investigation because the government had refused to release certain information.
This is thought to be intelligence material relating to the involvement of Russia in the case and of Litvinenko's work as an informant to MI6.
Sir Robert said last July that "sensitive" government evidence over the poisoning would now heard in closed sessions of the public inquiry.
The possible involvement of the Russian government in the murder would be of "central importance to my investigation", he said.
According to information that came out during the inquest, Litvinenko had been working for MI6 for several years during his time in London.
As part of this, Litvinenko also began assisting the Spanish security services. It is understood that his work in Spain involved investigating organised crime networks.
Litvinenko's work in Spain, as well as in Italy and Georgia, after leaving Russia and the KGB, has given rise to competing theories about who might have been behind his death.
