Particularly vexing to the United States and its allies is Moscow's refusal to endorse language that would tell Tehran it has no choice but to freeze uranium enrichment or face potential sanctions.
Moscow had previously signaled that it was ready to support a tough line.
Russian Foreign Minister Sergey Lavrov and counterparts from the United States, China, Britain, France and Germany agreed July 12 to resume Security Council deliberations after Tehran refused requests to respond by that date to their offer of rewards in exchange for an enrichment freeze and other nuclear concessions.
A statement on behalf of the six said they agreed to "seek a ... Security Council resolution which would make ... suspension mandatory."
Work on a resolution had been suspended May 3 to allow the six powers to draw up a plan of perks if Iran freezes enrichment and starts talks meant to secure its agreement to a long-term moratorium on the activity, or punishments that include the threat of selective U.N. sanctions if it doesn't.
While Iran argues it has a right under the Nuclear Nonproliferation Treaty to the technology for power generation, there is increasing international concern that Tehran wants to enrich uranium to weapons-grade levels for use in the fissile core of nuclear warheads.
Iran has not turned down the six-power offer but has shown no sign it is ready to give up enrichment. Tehran has said it will respond Aug. 22 to the package — a date rejected as too late by the six nations making the offer.
In a text made available to the AP Saturday, chief Iranian nuclear negotiator Ali Larijani repeated that enrichment belongs to "the inalienable rights of the Iranian nation" and warned his country would "reconsider its nuclear policies" if pressured too harshly — a possible threat to quit the Nonproliferation Treaty.
The text — remarks he made Thursday to Iran's Supreme National Security Council — was forwarded by Tehran to the Vienna-based International Atomic Energy Agency with a request that it be circulated among its 35 board member nations.
With the United States, Britain and France insisting that a freeze be made mandatory, Russian reluctance could seriously dent the show of unity of the six nations and damage their efforts to persuade Iran to compromise.
The diplomats — who demanded anonymity in exchange for discussing the substance of the dispute — told The Associated Press there was no indication what was dictating the apparent change in Russian tactics.
But it could be as simple as the belief by Moscow that Tehran would not give up its right to enrichment. If so, any resolution telling it do so and threatening penalties if it doesn't would escalate the confrontation — something the Russians fear could lead to military action.
A draft resolution drawn up by Britain and France and circulated last week among most members of the 15-nation council "decides" that Tehran "shall suspend all enrichment-related and reprocessing activities ... and suspend the construction" of its heavy water reactor, which can produce plutonium.
One of the diplomats said that wording reflected the agreement among Russia and the five other nations at the July 12 meeting in the French capital but Moscow now was trying to distance itself from the Paris declaration and seek a resolution which is not legally binding.
With the Russians opposed to any threat of armed force, the draft refers to Article 41 of Chapter 7 in the U.N. Charter. This allows punishments that do not involve the use of armed force, such as economic penalties, banning air travel or breaking diplomatic relations.
But if the demand on enrichment is anything less then mandatory, any ultimatum loses much of its meaning because there is little else concrete left to enforce — a temporary freeze is the "red line" for the United States and its Security Council allies on which they have been unwilling to compromise.
The diplomat said other differences — including Russian objections to describing Iran as a "threat to international peace and security" were close to being solved.