By Jonathan Marcus
BBC News diplomatic correspondent
The five United Nations Security Council permanent members - Britain, China, France, Russia and the United States, plus Germany - are due to debate new steps against Iran following its refusal to abandon the nuclear programme.
Iran's nuclear programme remained secret for years
This issue has dogged the international agenda for several years but there are signs that this latest round of diplomacy will not simply be business as usual.
Two things have happened that significantly affect the atmosphere surrounding the Iranian nuclear dossier.
On the one hand the pressure on Iran seems to have been relaxed somewhat by an agreement made between the International Atomic Energy Agency and the Iranians setting out a "work plan", under which Tehran has given an undertaking to resolve many uncertainties about its past nuclear activities.
This plan has especially angered the Americans, but the French, British and to an extent the German governments are far from enthusiastic either.
US Secretary of State Condoleezza Rice has openly criticised the IAEA's chief, Mohamed ElBaradei.
The charge is that, in effect, he is muddying the waters and giving the Iranians a means of appearing to cooperate, while avoiding the central thrust of two UN Security Council resolutions that demand that the Tehran government immediately halt its uranium enrichment programme.
The second change looks to be working in the opposite direction. This is the arrival of a new French President, Nicolas Sarkozy, who clearly wants to step-up the pressure on Iran and who clearly believes that action needs to go way beyond the lowest common denominator that the Security Council might accept.
Tougher sanctions sought
This has been the constant dilemma in pursuing the path of UN-backed economic sanctions against Tehran.
The US is critical of IAEA's head, Mohamed El Baradei
Consensus has inevitably been the name of the game with the essential need for the US and its allies to keep the Russians and the Chinese on board.
Part of the purpose of the meeting of senior officials in Washington is to clarify exactly where the Russians and the Chinese stand.
Western diplomats say that there are signs that both Moscow and Beijing are buying into the idea of Mr ElBaradei's "work plan".
This, they fear, risks putting the brakes on action at the Security Council level.
That is why the French, backed by the Dutch and others, are seeking tougher additional sanctions, perhaps through the European Union.
This idea is in its infancy. But it is bolstered by the assessment from Washington which argues that bilateral financial sanctions are indeed having an impact in Tehran.
The problem of course is that they are not having the effect the Americans want.
Iranian nuclear policy is not changing. Their research programme moves slowly forward, with diplomatic efforts to stop them always lagging some steps behind.
This is why the French in particular have sought to galvanise the debate by warning that the alternative to tougher sanctions may ultimately be military action.
The Iranians of course continue to insist that their programme is entirely peaceful in nature, aimed simply at generating electricity.
President Ahmadinejad has strongly defended Iran's nuclear programme
Iran's failure, though, to satisfy the Security Council may continue to delay the initial shipment of nuclear fuel to its Russian-built reactor.
One of the difficulties in all of this is that, leaving aside Iran's past behaviour and the sometimes bellicose noises from its president, this is a debate about a potential problem.
It is about Iran's capacity to master the nuclear fuel cycle, a process that would enable it to go down the weapons route if it so wished.
The existing non-proliferation machinery is not good at dealing with this kind of problem.
And the whole debate is further complicated by fears that more hawkish voices in Washington are using this issue to make the case for regime-change in Iran.
The shadow of Iraq and its alleged weapons programmes falls heavily over the current discussions.