Details of the aggressive surveillance operation, which involves interception of the home and office telephones and the emails of UN delegates in New York, are revealed in a document leaked to The Observer.

The disclosures were made in a memorandum written by a top official at the National Security Agency - the US body which intercepts communications around the world - and circulated to both senior agents in his organisation and to a friendly foreign intelligence agency asking for its input.

The memo describes orders to staff at the agency, whose work is clouded in secrecy, to step up its surveillance operations 'particularly directed at... UN Security Council Members (minus US and GBR, of course)' to provide up-to-the-minute intelligence for Bush officials on the voting intentions of UN members regarding the issue of Iraq.

The leaked memorandum makes clear that the target of the heightened surveillance efforts are the delegations from Angola, Cameroon, Chile, Mexico, Guinea and Pakistan at the UN headquarters in New York - the so-called 'Middle Six' delegations whose votes are being fought over by the pro-war party, led by the US and Britain, and the party arguing for more time for UN inspections, led by France, China and Russia.

The memo is directed at senior NSA officials and advises them that the agency is 'mounting a surge' aimed at gleaning information not only on how delegations on the Security Council will vote on any second resolution on Iraq, but also 'policies', 'negotiating positions', 'alliances' and 'dependencies' - the 'whole gamut of information that could give US policymakers an edge in obtaining results favourable to US goals or to head off surprises'.

Dated 31 January 2003, the memo was circulated four days after the UN's chief weapons inspector Hans Blix produced his interim report on Iraqi compliance with UN resolution 1441.

It was sent by Frank Koza, chief of staff in the 'Regional Targets' section of the NSA, which spies on countries that are viewed as strategically important for United States interests.

Koza specifies that the information will be used for the US's 'QRC' - Quick Response Capability - 'against' the key delegations.

Suggesting the levels of surveillance of both the office and home phones of UN delegation members, Koza also asks regional managers to make sure that their staff also 'pay attention to existing non-UN Security Council Member UN-related and domestic comms [office and home telephones] for anything useful related to Security Council deliberations'.

Koza also addresses himself to the foreign agency, saying: 'We'd appreciate your support in getting the word to your analysts who might have similar more indirect access to valuable information from accesses in your product lines [ie, intelligence sources].' Koza makes clear it is an informal request at this juncture, but adds: 'I suspect that you'll be hearing more along these lines in formal channels.'

Disclosure of the US operation comes in the week that Blix will make what many expect to be his final report to the Security Council.

It also comes amid increasingly threatening noises from the US towards undecided countries on the Security Council who have been warned of the unpleasant economic consequences of standing up to the US.

Sources in Washington familiar with the operation said last week that there had been a division among Bush administration officials over whether to pursue such a high-intensity surveillance campaign with some warning of the serious consequences of discovery.

The existence of the surveillance operation, understood to have been requested by President Bush's National Security Adviser, Condoleezza Rice, is deeply embarrassing to the Americans in the middle of their efforts to win over the undecided delegations.

The language and content of the memo were judged to be authentic by three former intelligence operatives shown it by The Observer. We were also able to establish that Frank Koza does work for the NSA and could confirm his senior post in the Regional Targets section of the organisation.

The NSA main switchboard put The Observer through to extension 6727 at the agency which was answered by an assistant, who confirmed it was Koza's office. However, when The Observer asked to talk to Koza about the surveillance of diplomatic missions at the United Nations, it was then told 'You have reached the wrong number'.

On protesting that the assistant had just said this was Koza's extension, the assistant repeated that it was an erroneous extension, and hung up.

While many diplomats at the UN assume they are being bugged, the memo reveals for the first time the scope and scale of US communications intercepts targeted against the New York-based missions.

The disclosure comes at a time when diplomats from the countries have been complaining about the outright 'hostility' of US tactics in recent days to persuade then to fall in line, including threats to economic and aid packages.