By Louis Charbonneau
The U.S. declaration came a day 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," said Richard Grenell, spokesman for the U.S. mission to the United Nations.
"They leave the Security Council no choice but to increase the sanctions, as called for in the last resolution passed."
Tehran has not formally responded to the offer. But Iranian President Mahmoud Ahmadinejad said on Saturday that Iran would not back down in its nuclear dispute with the powers, which have supported three rounds of U.N. Security Council sanctions.
"In whichever negotiation we take part … it is unequivocally with the view to the realization of Iran's nuclear right and the Iranian nation would not retreat one iota from its rights," Ahmadinejad said in a statement.
The West accuses Iran of seeking to develop nuclear weapons under cover of a civilian power program. Iran, the world's fourth largest oil producer, says its uranium enrichment drive is aimed solely at generating electricity.
A spokeswoman for EU foreign policy chief Javier Solana said in Brussels that he and Iran's top nuclear negotiator, Saeed Jalili, would discuss the six powers' offer soon.
"We expect a contact between Mr. Solana and Mr. Jalili soon by phone," the spokeswoman said. She gave no further details.
Western officials gave Tehran two weeks from July 19 to respond to their offer not to impose more U.N. sanctions on Iran if it froze any expansion of its nuclear work.
That suggested a deadline of August 2 but Iran, which has repeatedly ruled out curbing any of its nuclear activities, dismissed the idea of having two weeks to reply.
The five permanent Security Council members and Germany have appointed Solana to be their liaison with Iran.
RELUCTANCE AT SECURITY COUNCIL
The U.S. delegation at the United Nations might have to put some pressure on the rest of the council to discuss Iran again. Diplomats from some of the 14 other council members have said they would prefer not to enter into negotiations on another round of sanctions against Iran at the present time.
The third sanctions resolution was approved in March. Among the penalties it imposed were wider travel and financial restrictions on named Iranian individuals and companies.
One of the main reasons for council members' reluctance to take up Iran now is the upcoming U.S. presidential election in November and what it could mean for U.S. policy on Iran.
U.S. presidential candidate Barack Obama, a Democrat, has been highly critical of Republican President George W. Bush's handling of the Iran issue and has promised that if elected he would pursue a policy of greater engagement with Tehran.
Republican candidate John McCain has criticized Obama's suggestion that he would pursue direct talks with Tehran.
The other reason for the council's reluctance is that Russia and China do not want to discuss sanctions now. Diplomats say the two veto-wielding council members want to give Iran time to consider the offer of economic and political incentives in exchange for a suspension of enrichment.
Moscow and Beijing reluctantly backed all three rounds of U.N. sanctions against Iran but pushed hard to try to water them down beforehand in negotiations on the resolutions.
Still, council diplomats say, even China and Russia are frustrated with Tehran's open defiance of four Security Council resolutions demanding that it suspend enrichment, a technology that could be used to produce fuel for atomic weapons.
Separately, Syrian President Bashar al-Assad said in Tehran on Sunday that Damascus was not mediating or bringing a message from the West to Iran over its disputed nuclear plans but could play a role to help defuse the row in future.
Assad made his comments during a two-day trip to Iran that followed a visit to Paris in July when he told French President Nicolas Sarkozy he would use his good ties with Tehran to help resolve the atomic stand-off.
(Additional reporting by the Tehran bureau and Ingrid Melander in Brussels; Editing by Bill Trott)
