Unexpected Russian opposition to key parts of a US-backed Security Council draft resolution is threatening international unity on how to deal with Teheran's nuclear defiance, UN diplomats said Saturday. Particularly vexing to the United States and its allies is Moscow's refusal to endorse language that would demand Teheran freeze uranium enrichment or face potential sanctions. Russia had signaled that it was ready to support a tougher line. Russian Foreign Minister Sergey Lavrov delivered a veiled warning to Teheran on enrichment and its refusal to respond to an international offer to negotiate its nuclear program as recently as July 12, warning that the Security Council "will consider steps appropriate to the situation," unless the Islamic republic complied. He and counterparts from the United States, China, Britain, France and Germany agreed July 12 to resume Security Council deliberations after Teheran refused requests to respond by that date to an international package of incentives meant to persuade Iran to freeze enrichment and make other nuclear concessions. Work on a resolution was 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 is agreement to a long-term moratorium on the activity - or punishments that include the threat of selective UN sanctions if it doesn't. While Iran argues it has a right to the technology under the Nuclear Nonproliferation Treaty to generate power, there is increasing international concern that Teheran wants to enrich uranium to weapons-grade levels for use in the fissile core of nuclear warheads. With the United States, Britain and France insisting on a freeze, Russian opposition could seriously dent the unity of the six nations - the five permanent Security Council members plus Germany - that have been spearheading 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 Teheran would not give up its right to enrichment. If so, any resolution demanding 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 demands that Teheran "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 demand 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." "They now want to water down the text," said the diplomat.