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UNITED NATIONS, Aug 3 (Reuters) - The U.N. Security Council will have to increase sanctions on Iran for ignoring demands that it freeze sensitive nuclear activities, a spokesman for the U.S. mission to the United Nations said on Sunday.

U.S. spokesman Richard Grenell was speaking a day after after an informal deadline lapsed for Iran to respond to an offer from the United States, Britain, France, Germany, China and Russia for talks on its disputed nuclear program.

"It is clear that the government of Iran has not complied with the international community's demand to stop enriching uranium and isn't even interested in trying," Grenell said.

"They leave the Security Council no choice but to increase the sanctions, as called for in the last resolution passed," he added.

(Reporting by Louis Charbonneau; editing by Bill Trott)

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