The UN's nuclear watchdog agency says Iran has obtained black market documents that serve no purpose other than to help it make a nuclear warhead.

The International Atomic Energy Agency released a report Tuesday on the Iranian situation in advance of a Thursday meeting of its 35-member board.

However, the report also says Tehran has not started small-scale uranium enrichment since announcing it would back on Jan. 10.

The findings will still likely add to pressure to refer Iran to the UN Security Council.

Earlier Tuesday, a senior Iranian official cautioned any move to refer Iran's nuclear file to the UN Security Council would mean the end of diplomacy.

Leading Iranian nuclear negotiator Ali Larijani issued the sharply worded warning after an agreement by key powers to have the IAEA report Tehran to the council.

"Reporting Iran's dossier to the UN Security Council will be unconstructive and the end of diplomacy," said Larijani, who state television quoted as saying that Tehran believes the issue can be resolved peacefully.

Larijani's comments followed a London meeting that lasted into early Tuesday. The five permanent members of the Security Council agreed to recommend that the International Atomic Energy Agency (IAEA) report Iran to the Council.

An unnamed American diplomat told Reuters news agency: "This is the most powerful message we could have hoped for."

The key powers also decided the council should wait until March to take up the nuclear file after the IAEA's formal report on Tehran's activities.

Meanwhile, diplomats told The Associated Press that Iran gave the UN's nuclear watchdog sensitive documents that seem linked to nuclear warhead design.

Vice President Gholamreza Aghazadeh, who also runs Iran's Atomic Energy Organization, said it was difficult to predict the outcome of the IAEA meeting, the semi-official Iranian Students News Agency reported.

"The biggest problem for the West is that they can't find any (legal) justification to refer Iran to the UN Security Council," ISNA quoted him as saying.

Last week, Larijani flew to Moscow and Beijing to seek Russian and Chinese support against Western efforts to refer Iran to the Security Council.

Both Russia and China decided to vote for referral, a move that surprised observers as both nations have major economic ties to Iran and have been reluctant to escalate the case.

A French government official, speaking on the customary condition of anonymity, told AP that the Russian and Chinese ministers had been convinced of the need to show a united front.

Meanwhile, Russian and Chinese diplomats are expected to visit Tehran soon to urge Iranian officials to cooperate with the UN nuclear watchdog.

The United States suspects Iran is aiming to build atomic weapons, a charge with Tehran denies, saying its nuclear program is only for generating electricity.

Iran broke IAEA seals at a uranium enrichment plant Jan. 10 and said it would resume small-scale enrichment.

The decision provoked an outcry as enrichment is a process that can produce material that can be used in nuclear weapons as well as power plants.

If Iran's nuclear file is referred to the Security Council on Thursday, Iran could retaliate.

Iran has approved a law requiring the government to stop all voluntary cooperation with IAEA in the event of referral and would likely implement that law immediately.