The Bush administration warned Iran on Monday that the United States would consider taking action in the United Nations if Tehran did not respond by July 12 to an offer designed to halt uranium enrichment.

That is an important date because Secretary of State Condoleezza Rice will be holding talks with the foreign ministers of Britain, France, Germany, Russia and China, Undersecretary of State Nicholas Burns said.

While the United States still expects an answer this week, Burns said, he stressed the July 12 meeting as pivotal. If Iran has not responded by then, "then I think the pressure will be enormous on the Iranians from all the international community," he said.

"We will have to draw our own conclusions and probably consider some of the measures that have to do with action by the Security Council because by then Iran will have had five weeks to respond to the offer that was originally made back on June 1," Burns said in an interview on C-Span that will be aired Sunday.

The U.S. and its partners have offered Iran concessions in exchange for a verifiable suspension of uranium enrichment. The offer includes civilian nuclear technology from the U.S. and having the U.S. join Britain, France and Germany in now-suspended negotiations with Iran.

Iran has two paths to consider, Burns said: "Iran could sit down on a rational basis and talk to the countries about all these issues. Or it could face another path forward, and that would be action of the Security Council."

Pressed, Burns declined to say whether there was a consensus to impose economic or political sanctions on Iran. He said until Iran responded, "it wouldn't make much sense to answer a hypothetical question."

But he said frustration with Iran was growing. "Our sense, our hope, is that the Iranians will understand that they are fairly isolated diplomatically," Burns said.

Iran has offered to talk to the United States about the turmoil in Iraq, its neighbor. Burns said, "We'll have those talks when we think it is in our interest to have them."

Right now, he said, the Bush administration was focused on the nuclear question. "We want to get a response from the Iranian government on that question first," he said.