The head of the arms control unit at Russia's foreign ministry, Vladimir Yermakov, called on UN members to not "keep silence in hope that the situation will somehow blow over".

Addressing the preliminary review meeting of the Nuclear Non-Proliferation Treaty (NPT), Yermakov described the landmark 2015 Iran nuclear agreement as "quite a fragile compromise".

US President Donald Trump has fiercely criticised the complex deal and has threatened to ditch it completely if the signatories do not sign up to tougher measures against Iran.

The president is supposed to waive US sanctions against Tehran by a May 12 deadline to keep the agreement alive but if he refuses to do so, it may unravel.

Russia and China, both supporters of the pact known as the Joint Comprehensive Action Plan (JCPOA), drafted a statement affirming their "unwavering support for the comprehensive and effective implementation" of the deal.

Yermakov urged all nations at the UN nuclear meeting to sign on.

"We believe there is a demand for such a collective message by the (meeting) and hope that the document will find broad support," he said.

The Russian official then took a thinly veiled shot at Trump's call to renegotiate the JCPOA in hopes of securing tougher terms.

"Any attempts to amend (the) text for someone's benefit will inevitably... have powerful negative consequences for regional global stability and security," Yermakov said.

- No renegotiation -

In his own speech to the forum, Iran's ambassador to the International Atomic Energy Agency Reza Najafi took on Trump directly, rejecting what he described as Washington's "ultimatum to certain JCPOA participants for one-sided alterations of the deal's provisions".

"Our response to that threat is clear and firm: No, the JCPOA will not be renegotiated or altered," Najafi said, echoing a stance adopted by Tehran after Trump first raised the prospects of re-visiting a pact negotiated by his predecessor Barack Obama.

The joint Russia-China text was introduced at the UN meeting after Moscow's Foreign Minister Sergei Lavrov vowed in Beijing Monday that China and Russia would block any attempts to "sabotage" the Iran nuclear agreement.

During a first day of discussions on the NPT in Geneva Monday, a long line of speakers had voiced their support for the Iran deal, including the UN's top representative for disarmament affairs Izumi Nakamitsu.

European leaders are also scrambling to the save the deal.

French President Emmanuel Macron is in Washington this week lobbying Trump to preserve the pact, with German Chancellor Angela Merkel due in the US capital on Friday.