Image caption The NSA collects records of every US phone call made and received

The White House is urging Congress to reject an attempt to stop the National Security Agency (NSA) collecting Americans' phone records.

With a key vote coming up, President Barack Obama's spokesman said curbs on the NSA would "hastily dismantle" a vital counter-terrorism tool.

NSA chief Gen Keith Alexander spent Tuesday lobbying Congressmen to vote against the proposed measure.

Critics say NSA phone data collection is an unwarranted invasion of privacy.

The details of the programme were made public by Edward Snowden, who had worked for the NSA and is now a fugitive, awaiting a decision on his asylum application in Moscow.

'Blunt approach'

House of Representatives Republican Congressman Justin Amash has introduced an amendment to a defence spending bill, which would block funding for the NSA's programme to collect details of every call made by or to a US phone.

How the US collects phone records The National Security Agency (NSA) began collecting Americans' phone records in 2001, as part of far-reaching surveillance programmes launched by then-President George W Bush in the aftermath of the 9/11 attacks. But the scope of the practice, continued under President Barack Obama, only became apparent in June when ex-CIA contractor Edward Snowden leaked classified US surveillance files. It emerged that a US secret court had ordered phone company Verizon to hand over to the NSA the phone records of tens of millions of American customers. This information, known as metadata, includes the numbers of the originating and receiving phone, the call's duration, time, date and location (for mobiles, determined by which mobile signal towers relayed the call or text). The contents of the conversation itself, however, are not covered. The surveillance applies to calls placed within the US, and calls between the US and abroad. Q&A: Prism internet surveillance

Mr Amash said: "My amendment blocks funding of NSA's collection of personal data if that data does not pertain to a person under investigation."

A vote on the amendment will take place on Wednesday.

Even if it passes the House, the measure is seen as unlikely to become law, as Mr Obama's Democratic Party controls the Senate.

House Democrats John Conyers and Jared Polis supported the move, which they said "makes sure that innocent Americans' information isn't needlessly swept up into a government database".

However, the White House said the measure would deprive the intelligence agencies of a key tool in preventing terror attacks.

"This blunt approach is not the product of an informed, open or deliberative process," said spokesman Jay Carney.

"We urge the House to reject the Amash amendment, and instead move forward with an approach that appropriately takes into account the need for a reasoned review of what tools can best secure the nation."

Party split

NSA head Gen Alexander held separate, closed-door sessions with Republicans and Democrats on the eve of the vote in an attempt to persuade them to vote against.

Afterwards he told CNN: "What you can see is that everybody wants to ensure we protect civil liberties and privacy and defend this country.

"We have that responsibility, and the issue is, how do we do that? How do we take care of our people and protect our civil liberties and privacy? This is a tough issue."

The issue has split the main parties, with Democrats and Republicans lining up on both sides of the debate.

Meanwhile it is thought possible that Russia will announce whether Edward Snowden has been successful in his application for temporary asylum in the country.

Image caption NSA chief Gen Keith Alexander has pleaded for the ability to collect phone data

He has been in limbo in a Moscow airport for weeks, but his lawyer Anatoly Kucherena says Wednesday is the deadline for the Federal Migration Service to rule on his request.

A number of Latin American states say they are willing to offer the former intelligence systems analyst asylum, but he says he first needs Russian asylum in order to be able to travel, since the US has cancelled his passport.

The US has charged Mr Snowden with leaking classified information.

Mr Putin has refused to hand him to US authorities, but says he can only stay in Russia if he stops leaking secrets about US surveillance schemes.