The United States and Europe criticized Iran on Wednesday for its defiance of a U.N. Security Council demand that it stop uranium enrichment, and the six world powers prepared for a strategy session on how to persuade Tehran to comply.

Delegates voiced criticism of Tehran at a conference seeking ways to improve the Nuclear Nonproliferation Treaty in its first full day of substantive business after a week of deadlock over Iran's opposition to language of the agenda ended Tuesday.

The issue stalling the meeting had been Tehran's refusal to accept a phrase calling for the "need for full compliance with" the pact.

Diplomats accredited to the conference said Iran felt that wording would allow it to be targeted for its defiance of U.N. Security Council demand that it suspend all activities linked to enrichment, which can be used to generate power as well as create fissile warhead material.

The South African compromise accepted Tuesday footnoted that phrase to the agenda to reflect that all aspects of the treaty must be fully observed — an allusion to the need for the United States and other nuclear weapons states to disarm.

The Nuclear Nonproliferation Treaty calls on nations to pledge not to pursue nuclear weapons in exchange for a commitment by five nuclear powers — the U.S., Russia, Britain, France and China — to move toward nuclear disarmament.

While some delegations touched on disarmament commitments Wednesday, the U.S. and the European Union as well as several powerful EU nations nonetheless focused on Tehran's refusal to heed the Security Council.

"The United States remains gravely concerned by violations by Iran ... of the treaty's core obligations," chief U.S. delegate Christopher A. Ford told the meeting, alluding to findings by the International Atomic Energy Agency interpreted as contravention of the pact. And he warned of signs that Tehran might be "considering withdrawal" — like North Korea did in 2002 in a move that culminated in Pyongyang's test of a nuclear weapon last year.

German chief representative Ruediger Luedeking, speaking for the EU, deplored "Iran's failure to comply" with Security Council resolutions demanding an enrichment freeze.

Separately, EU nations France and Britain also criticized Tehran. British chief delegate John Duncan spoke of "strong and increasing concerns about the proliferation implication of Iran's nuclear program," and Jean-Francois Dobelle of France demanded that Iran "suspend without delay" enrichment activities and other programs that could be used to make weapons.

Iran argues it is entitled to enrich under the treaty provision giving all pact members the right to develop peaceful programs. But suspicions bred by nearly two decades of clandestine nuclear activities, including questionable black-market acquisitions of equipment and blueprints that appear linked to weapons plans, have led the U.N. Security Council to impose sanctions over Tehran's refusal to halt its enrichment program. The enrichment process can be used for generating energy or producing the fissile core of nuclear warheads.

As the Vienna meeting ended its Wednesday session, senior officials from the U.S. and five other world powers prepared for talks on options left to break Iran's resistance to suspending enrichment.

U.S. Deputy Secretary of State Nicholas Burns was to meet Thursday in Berlin with counterparts from Russia, Britain, France as well as Germany, and a representative from the European Union, with a senior Chinese official participating by telephone, organizers said.

"The central point of the talks should be how we can prepare together the next talks" between top EU foreign policy official Javier Solana and Ali Larijani, Iran's chief nuclear negotiator, said German Foreign Ministry spokesman Martin Jaeger.

After meeting late last month, Solana and Larijani spoke of some progress and agreed to meet again within weeks in hopes of finding enough common ground to serve as the basis of renewed talks between Tehran and the six powers — the five permanent Security Council members plus Germany.

Diplomats told the AP after those talks that the concept of a "double time out" was touched upon by the two men in their last discussion — with the two sides setting the stage for substantive discussions by Iran freezing its enrichment program in exchange for a commitment of no new U.N. sanctions.

Still, Iran continues to publicly rule out freezing enrichment as a precondition for negotiations. Instead it appears to be expanding its program

Diplomats last week told the AP that Iran had recently set up more centrifuges at its underground uranium enrichment plant at Natanz, bringing the number of machines ready to spin uranium gas into enriched form to more than 1,600.

Its ultimate goal is to have 50,000 centrifuges. That would be enough to supply fuel for what Iran says is a planned network of atomic reactors to generate electricity. Or it could produce material for a full-scale nuclear weapons program.