Both nations object to the use of Chapter 7 of the UN Charter, used in dozens of Security Council resolutions for peacekeeping missions and other legally-binding actions.
Although Chapter 7 allows for sanctions and even war, a separate resolution is required to specify either step.
Moscow and Beijing, which have veto power, fear that too much pressure on Iran would be self-defeating or precipitate an oil crisis. Both worry that the US would use a Chapter 7 resolution to justify military action.
Wang Guangya, the Chinese ambassador to the UN, said: "I think we have serious difficulty with Chapter 7 and the threat to international peace and security. These are the basic ones."
He was referring to a paragraph in the resolution's preamble that indicates that Iran's nuclear programme was a "threat to international peace and security".
Wang said both provisions should be removed, even though Chapter 7 is basic to France and Britain, which drafted the resolution, and the US, which backs it.
Wang: We have serious
difficulty with Chapter 7
Vitaly Churkin, the Russian ambassador to the UN, said the main purpose of the resolution should be to back the International Atomic Energy Agency, the UN nuclear watchdog.
"It's clear this resolution is not about sanctions because they are not in the resolution," he said. "It is clear that this resolution is not providing legal ground for the use of force. Everybody agrees on that."
Churkin said the use of Chapter 7 "might in fact detract from the strength of this resolution because [it] might be detracting from our goal of supporting the IAEA in its activities in working with Iran."
The resolution, introduced on Wednesday, would compel Iran to suspend nuclear enrichment. It does not call for any other action if Iran does not comply, but the US has made it clear that sanctions would be the next step.
The draft also says the Security Council "expresses its intention to consider further measures as may be necessary to ensure compliance", a veiled threat of sanctions without imposing them.
Negotiations now concern formulas that would make the resolution legally binding but exclude any hint of the use of force, diplomats said.
John Bolton, the US ambassador to the UN, said: "The issue whether there is another way that is acceptable is something that we have asked the Russians and the Chinese to provide. We are waiting to hear how one might do that."
Bolton had wanted an agreement before foreign ministers from Germany and the five permanent Security Council members - the US, Britain, France, Russia and China - meet on Iran late on Monday. This now seems unlikely, council members said.
However, all 15 council nations will discuss the resolution on Saturday at Britain's UN mission.
The Security Council issued a non-binding statement in March asking Iran to abandon uranium enrichment, a process than produce fuel to generate electricity or material for weapons.
The council asked for a report within 30 days from the IAEA, whose director, Mohammed ElBaradei, said on April 28 that Iran had not complied.
"The issue whether there is another way that is acceptable is something that we have asked the Russians and the Chinese to provide. We are waiting to hear how one might do that."
John Bolton, US ambassador to the UN
Iran says its activities are legal and peaceful. It recently accelerated its pace of uranium enrichment but remains far below levels needed to make an atomic bomb.
Iranian officials note that the IAEA has not found a weapons programme after three years of scrutiny.