The meeting in Vienna of the International Atomic Energy Agency adjourned its plenary session until further notice in order to allow for informal talks, IAEA spokesman Mark Gwozdecky said.

"There is a lot of hard negotiating going on," IAEA chief Mohamed ElBaradei told reporters.

IAEA officials said that a resolution might be brought before the IAEA's 35-nation board of governors on Friday, but this was not certain.

The United States is pushing for the atomic agency to adopt a resolution at a meeting in Vienna this week that would set a deadline, possibly as early as October 31, for Tehran to fully suspend uranium enrichment and to take other measures, diplomats said.

"We want the resolution to lay out essential and urgent steps for Iran to take," a US official said.

He said the United States saw the deadline as a "trigger," so that if Iran failed to do what was asked, the IAEA would automatically at its next meeting in November take Tehran to the UN Security Council for possible sanctions.

A non-American western diplomat said Washington was pushing for a tougher resolution than one drafted by Britain, France and Germany, who have stressed constructive engagement, rather than confrontation, with Iran.

Their resolution gives Iran a November deadline to allay concern that it is secretly making atomic weapons but does not say that Iran should automatically be taken to the Security Council if it fails to do this.

Non-aligned states were firmly in support of the European position.

Malaysia's IAEA ambassador Hussein Haniff said they "do not want to see a trigger mechanism becuase that is pre-emptive."

He said the IAEA should work from reports by its director general ElBaradei and "there is nothing in the report that calls for Iran to be referred to the UN Security Council."

Meanwhile, Iran appeared to be hardening its stance, saying it would not agree to an unlimited suspension of uranium enrichment, a process that makes what can be fuel for civilian reactors or the explosive core of atomic bombs.

Hossein Mousavian, the head of the Iranian delegation to the IAEA meeting, warned that "we will not accept any bargaining for an unlimited suspension."

"Iran will not accept having to make new commitments that extend the scope of the suspension of uranium enrichment," he said.

Tehran insists its program is strictly for civilian purposes and within the confines of the nuclear Non-Proliferation Treaty (NPT).

The United States however maintains Iran has not lived up to an agreement a year ago to suspend the building of centrifuges used for enriching uranium and is in fact conducting a covert program to produce nuclear weapons.

ElBaradei warned against a deadline, saying he had not yet decided whether Iran's program had peaceful intent or was designed to develop weapons.

"There is no artificial deadline whereby I can say in November, I can promise that everything will be completed," he told reporters.

"Have we seen any proof of a weapons program? Have we seen undeclared enrichment? Have we seen undeclared material?" he asked.

"Well, obviously on these issues until today there is none of that, but are we in a position to say that everything now is peaceful? Obviously we are not at that stage."

He said the investigation was "very complex" as it depended on information from both Iran and countries involved in the international black market that supplied Tehran with nuclear materials.

ElBaradei said he was ready to give an "evaluation" to the IAEA board of governors that would decide on a deadline, "but I am giving advance warning that this will not be the end of the story."

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