Secretary of State Mike Pompeo announced on Wednesday that the United States would put forward its long-awaited resolution despite ardent opposition from Russia and China.
But UN diplomats say opposition to the resolution's current form is so widespread that Washington is unlikely even to secure the nine votes required to force Moscow and Beijing to wield their vetoes.
"The resolution takes a maximalist position on Iran," one diplomat told AFP.
Another said the draft "goes beyond the current provisions" of the ban on conventional weapons sales to Iran that ends on Oct 18.
The embargo is due to expire under the terms of a resolution that blessed the Iran nuclear deal, signed in July 2015 and officially known as the Joint Comprehensive Plan of Action (JCPOA).
Under the deal, negotiated by then US President Barack Obama, Iran committed to curtailing its nuclear activities for sanctions relief and other benefits.
President Donald Trump pulled America out of the accord in May 2018 and slapped unilateral sanctions on Iran under a campaign of "maximum pressure."
Iran has since taken small but escalating steps away from compliance with the nuclear accord as it presses for sanctions relief.
European allies of the United States - who along with Russia and China, signed the deal with Iran - have voiced support for extending the conventional arms embargo but their priority is to preserve the JCPOA.
The US text, seen by AFP, effectively calls for an indefinite extension of the embargo on Iran and uses hawkish rhetoric.
Diplomats fear the resolution threatens the nuclear agreement. Iran says it has the right to self-defense and that a continuation of the ban would mean an end to the nuclear deal.
"The focus should remain on preserving the JCPOA," a third diplomat told AFP.
"It is the only way to provide assurances about the exclusively peaceful nature of Iran's nuclear program. No credible alternative to this instrument has ever been proposed since the US withdrawal," they added.
Experts say the gulf between the US and its allies threatens a summer of discontent at the Security Council as the Oct 18 deadline approaches.