Iran on Monday rejected mounting international calls for it to sign an additional protocol of the Non-Proliferation Treaty (NPT) that would allow tougher inspections of its suspect nuclear programme.

The refusal came after Russia, which is helping the Islamic republic build its first atomic power plant in Bushehr in southern Iran, joined calls for Tehran to grant International Atomic Energy Agency (IAEA) inspectors full access to its nuclear facilities.

A string of nations, including Britain, France and Australia, have urged Iran to take what they call a badly-needed "confidence building" step that would ease jitters in Washington that the Islamic republic could go nuclear.

Iran has been clearly warned that crossing the nuclear weapons threshold would be "unacceptable", while its assertions that the programme is exclusively civil have failed to convince.

"If the Russians are worried, we are ready to discuss this with them," foreign ministry spokesman Hamid Reza Asefi told reporters, the day after Russia's Foreign Minister Igor Ivanov joined calls for Iran to show greater transparency.

"The question of sanctions has to be resolved first. We will not sign any other international accord while the West does not respect its obligations outlined by the NPT, and does not help us with peaceful nuclear technology as the NPT obliges them to," he added.

Iran, a signatory of the NPT, is currently only subject to IAEA inspections of declared sites. But the country has consistently argued that it has no obligation to grant more powers to the IAEA when other signatories are refusing to meet their NPT obligations related to the transfer of civil nuclear technology.

Russia is also coming under almost daily pressure from the United States to halt its multi-billion dollar nuclear cooperation with Iran, a country lumped into an "axis of evil" by US President George W. Bush.

But Asefi asserted that Moscow "has commitments with us that it has to respect".

As for US concerns, Asefi brushed them off as "pretexts".

"The United States is not really worried about what they call weapons of mass destruction or our nuclear programme. These are just pretexts: if they are worried, all they have to do is come here and help us build our nuclear power stations," he said.

And Asefi dismissed the argument that Iran has no need for nuclear power, given its massive oil and gas reserves - a logic given by the United States as a clear indication that the atomic energy programme here is just a convenient cover.

"It was the United States that proposed to the former regime of the Shah to build nuclear power plants," Asefi asserted.

But diplomats here say the pressure that Iran now finds itself under is only likely to increase over the coming months.

A key test will be a mid-June report on Iran's programme by IAEA chief Mohamed ElBaradei. Analysts believe he will conclude that his limited access to sites here leaves him unable to categorically state that Iran has no nuclear weapons programme.