The tension was underlined at the weekend as the Liberal Democrat leader, Paddy Ashdown, said he had been told of US plans to press Britain to put its Trident nuclear system on the NPT negotiating-table.
"The NPT has never been more important than it is now, with access to technology waxing, Cold War disciplines waning and rogue states hungering for nuclear arms," maintains John Holum, director of the United States Arms Control and Disarmament Agency. "But we are engaged in a real struggle.''
The NPT came into force 25 years ago, to prevent the spread of nuclear weapons beyond the five acknowledged "weapons states" - Britain, the US, Russia, France and China. Its supporters claim it has given the world insurance against disaster by compelling signatory nations to submit to inspections and refrain from producing nuclear weapons. The NPT provided the international legal basis for action to curb the nuclear aspirations of Iraq and North Korea.
There are now 170 signatories out of 185 members of the United Nations. But the treaty's term is up. It must be renewed at a conference opening in April, with a vote by mid-May. Britain, the US, Russia and France need to win a simple majority - 86 national votes - to extend it indefinitely. But that is proving far from simple.
"A number of countries are attracted to the self-defeating idea that the NPT should be held hostage," Mr Holum said. They want to make its extension conditional on a new test-ban treaty - which may be agreed within a year or so - or upon guaranteed progress towards disarmament.
But objections to automatic renewal do not stop there, as Mr Holum acknowledges. "Others, like Iran, think it should be amended to make access to nuclear technology an automatic right of parties - as if we had no memory of what happened in Iraq - and, indeed, no clue about Iran itself," he said.
Iran has emerged as the standard-bearer for a host of Third World objections to the dominance of the weapons states and their right to dictate what weapons systems should be available to governments.
The struggle is going on at the Conference on Disarmament in Geneva, where diplomats are engaged in negotiating a new comprehensive test-ban treaty, while lobbying behind the scenes for votes at the NPT showdown in April. Iran's ambassador to the UN in Geneva, Sirous Nasseri, has waged a skilful campaign to promote causes vital to Tehran.
President Ali Akbar Hashemi Rafsanjani recently pronounced that "destructive and anti-human nuclear weapons are against our culture, ideology and political viewpoint". Mr Rafsanjani complained that "America, in order to cover up the nuclear capability of the Jerusalem-occupying regime [Israel], which boldly refuses to sign the NPT, accuses Iran, which does not need nuclear weapons, of trying to gain access to such weapons."
Despite Mr Rafsanjani's protestations, Western intelligence services and foreign ministries have concluded that Iran does have a clandestine nuclear-weapons programme, but it is still far away from acquiring a nuclear device. Recent statements suggesting the contrary by the US Defense Secretary, William Perry, and the Foreign Secretary, Douglas Hurd, were intended to step up the pressure on Tehran in response to Mr Nasseri's manoeuvring in Geneva.Iran has thus succeeded in defining the broader political weakness of the NPT with disturbing clarity.
Arab countries led by Syria and Egypt are now arguing they should not sign on for renewal while Israel is not a signatory at all. Israel's Foreign Minister, Shimon Peres, responded this week that Israel would not give in to Iranian threats.
The stand-off in the Middle East mirrors a series of potentially flash- points elsewhere. Among other important countries that have not signed the NPT are India and Pakistan, which are both believed to be capable of waging limited nuclear war. The agreement between North Korea and the US to restrict Pyongyang's nuclear plans is riddled with uncertainty.
Western nations remain concerned about weapons and nuclear-material stockpiles in the fragmented states of the former Soviet Union. Only in South America is there real progress, with a guarantee by Argentina to accede to the treaty.
Diplomats are working on ways to win confidence and votes for an indefinite extension. The weapons powers are ready to assure non-nuclear nations against the use or threat of use of nuclear weapons.
Britain's chief negotiator in Geneva, Sir Michael Weston, has told the House of Commons Foreign Affairs Committee that he believes support for an indefinite NPT extension can be won by "an overwhelming majority." But as diplomats calculated the arithmetic in Geneva last week, the best bet was for 60 firm votes in favour of indefinite extension, with 15 or 20 waverers. And if only 86 out of 170 signatories renew the treaty, it would be a pretty tattered document.
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