The United States said Thursday that "time is wasting" in seeking a solution to the dispute over Iran's nuclear activities as Tehran has failed to respond constructively to Western proposals.
"We believe there is still time and space for diplomacy," U.S. Ambassador Susan Rice told a UN Security Council meeting on Iran's nuclear program.
"However, the onus is on Iran to respond constructively," Rice said, adding that Washington is seeking a "clear, united resolution" to Iran's uranium enrichment program, which she said has advanced beyond the 20-per-cent enrichment needed for civilian use of nuclear energy.
Iran has regularly insisted that it has no intention of creating a nuclear weapons program.
Rice said the five permanent council member countries - the U.S., Russia, China, France and Britain - plus Germany have proposed reciprocal steps aimed at solving the nuclear dispute. She said Iran has so far not responded.
The warning comes as the U.S. and allies are carrying out massive naval drills in the Gulf, which observers say are related to mounting tensions with Iran.
The maneuvers, codenamed IMCMEX-12, involve clearing mines that may be deployed by an hypothetical "enemy" to block shipping traffic, according to military commanders.
Arab countries, mainly Saudi Arabia, as well as Britain and France are participating in the maneuvers, expected to run until later this month.
Iran itself has said it will hold stage a major defense exercise next month.
Previously, Tehran has warned it would close the Strait of Hormuz if attacked. An estimated 20 per cent of global oil exports pass through the strait.
Prime Minister Benjamin Netanyahu has recently prodded the administration of President Barack Obama to draw "red lines" for Iran over its disputed nuclear program.
Netanyahu demanded in a round of interviews at the weekend that Washington and the rest of the world tell Tehran that, if Iran crossed a certain point, military force would be used against it.
But Obama, seeking re-election, said he would not impose red lines or a deadline on Tehran.
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