* Rice tells Iran to decide on nuclear deal
* Brown determined to prevent Iranian nuclear arms programme
* Iranian official says U.S. undecided on diplomacy vs force
ABU DHABI, July 21 (Reuters) - U.S. Secretary of State Condoleezza Rice warned Iran on Monday that it faced more sanctions if it defied a two-week deadline to agree to curb its nuclear programme.
Rice said Iran was stalling and must give a "serious answer" within the deadline set by six world powers, which offered trade and technical incentives if Tehran halts its uranium enrichment. The West fears Iran wants to build a nuclear bomb.
"We are in the strongest possible position to demonstrate that if Iran does not act then it is time to go back to that (sanctions) track," Rice told reporters before arriving in Abu Dhabi en route to Asia.
It was her first comment on the subject since Washington broke from usual policy and joined nuclear talks with Iran in Geneva on Saturday.
In Jerusalem, British Prime Minister Gordon Brown warned Iran of growing isolation if it rejected the offer from the major powers.
"Iran has a clear choice to make: suspend its nuclear programme and accept our offer of negotiations or face growing isolation and the collective response not just of one nation but of all nations around the world," Brown said.
He said Britain would continue, with its European partners and the United States, "in our determination to prevent an Iranian nuclear weapons programme".
France's foreign ministry also said the "Iranian regime must choose between the path of cooperation and the prospect of growing isolation," adding if Tehran pursues its enrichment activities "it will have to face new sanctions".
Rice said the United States would impose more bilateral sanctions on Iran and the Europeans would look at what they could do if Iran failed to meet the world powers' demand.
"The main thing is we will have to start considering what we do in New York," she said, referring to the U.N. Security Council which has imposed three rounds of sanctions on Iran.
Envoys from the United States, Russia, China, France, Germany and Britain -- the so-called sextet of world powers -- attended the Geneva meeting as well as Iran.
Iran's top nuclear negotiator, Saeed Jalili, said at the next meeting Iran would not discuss the demand to freeze its sensitive atomic work which the West fears is aimed at making bombs. Iran says its aims are peaceful.
Iranian President Mahmoud Ahmadinejad gave an upbeat assessment on Sunday. "Any negotiation that takes place is a step forward," he told reporters, according to IRNA news agency.
A senior Iranian official said Iran was ready to respond to any positive U.S. overture but it was unclear whether Washington had decided between diplomacy and force.
The U.S. government was "indecisive about whether to lean on diplomacy or the military option", said Deputy Foreign Minister Alireza Sheikh-Attar, according to the student news agency ISNA on Monday.
Rice said Iran's envoy to Saturday's talks, attended by senior U.S. diplomat William Burns, engaged in small talk rather than address the central issue of the sextet offer.
"I understand that it was at times meandering," Rice said.
She said EU foreign policy chief Javier Solana "clarified" Iran's choices at the talks.
"It was also a very strong message to the Iranians that they can't go and stall and make small talk and talk about culture and that they have to make a decision," said Rice.
Burns will brief Rice during her one-night stopover in Abu Dhabi, where she will meet foreign ministers from the United Arab Emirates, Egypt, Jordan and Iraq as well as senior officials from Bahrain, Qatar, Saudi Arabia and Kuwait.
U.S. attendance at the Geneva talks was an about-face and comes as Washington is considering whether to open an interest section in Tehran, which would allow for diplomatic contact while falling short of diplomatic ties.
Rice said such a move should not be seen as a thawing of relations. Washington broke ties with Iran nearly 30 years ago.
Rice said there were no plans to join further nuclear talks unless Iran met conditions to give up the enrichment work.
"I think we have done enough to demonstrate that the United States is serious and to assure our partners that we are serious and to show the Iranians that we are serious. I think we have done enough," she said. (Additional reporting by Adrian Croft in Jerusalem and Francois Murphy in Paris) (Editing by Stephen Weeks)
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