Unexpected Russian opposition to key wording of a U.S.-backed Security Council draft resolution is straining international unity on how to deal with Iran's nuclear defiance, U.N. diplomats said Saturday.

The apparent change of heart is the latest obstacle in the months-long attempt to pressure Iran's hardline Islamic government to suspend uranium enrichment, which many countries fear Tehran wants to use for a nuclear program.

Iran argues its needs enrichment to make energy and is entitled to it under the Nuclear Nonproliferation Treaty .

Although it initially urged restraint, Russia, as recently as July 12 in Paris, signaled it was ready to support a tougher line. Russian Foreign Minister Sergey Lavrov and counterparts from the United States, China, Britain, France and Germany agreed then to resume Security Council deliberations after Iran refused requests to respond to an international offer to negotiate its nuclear program.

A statement on behalf of the six said they agreed to "seek a ... Security Council resolution which would make ... suspension mandatory."

CountryWatch: Iran

The diplomats, who spoke on condition of anonymity due to the sensitive nature of the matter, say particularly vexing is Moscow's refusal now to endorse language that would tell Tehran it has no choice but to freeze uranium enrichment or face sanctions.

Work on a resolution was suspended May 3 to allow the six powers to devise incentives for Iran to freeze enrichment and start talks meant to secure its agreement to a long-term moratorium on the activity, which can produce material for use in the fissile core of nuclear warheads as well as fuel for reactors.

The incentives include advanced technology and the easing of U.S. sanctions on the sale of aircraft and aircraft parts. The United States, breaking with decades of policy, has said it is willing to join in the multinational talks

Iran has not turned down their 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 which the six nations extending the offer have rejected as too late.

In remarks made available to The Associated Press 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 NPT.

The remarks were made Thursday to Iran's Supreme National Security Council and were forwarded by Tehran to the Vienna-based International Atomic Energy Agency with a request that it be circulated among members of its 35-nation board.

One of the diplomats said Russia seems to be distancing itself from the Paris declaration and is seeking a resolution that is not legally binding.

The diplomat said other differences — including Russian objections to describing Iran as a "threat to international peace and security" were close to being solved.

Russia's reluctance could seriously undermine efforts to secure a compromise from Iran, especially as the United States, Britain and France insist that the freeze be made mandatory.

The West wants an Aug. 31 deadline for Iran to comply on the freeze demand. But if that demand is anything less then mandatory, any ultimatum loses much of its meaning because there is little else concrete left to enforce.

While the diplomats told the AP there was no indication what brought about the apparent change of heart, it could be as simple as Moscow believing that Iran will not give up its right to enrichment. Any resolution demanding this and threatening sanctions, therefore, is something the Russians fear could lead to military action which they oppose.

The wording of a draft resolution drawn up by Britain and France, and circulated last week among most of the 15-members of the council, "decides" that Tehran "shall suspend all enrichment-related and reprocessing activities ..."

In a nod to Russia's resistance to the military option, the draft also 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.