Washington ? The United States and Europe are poised to seek harsher U.N. financial sanctions against Iran if it fails to meet this weekend’s deadline to accept an international offer of negotiations in exchange for freezing its nuclear program, diplomats said Friday.

It’s uncertain how, or even whether, Iran will formally respond to the offer. Authorities in Tehran have given no sign that they’re willing to accept the offer of a “freeze for a freeze” – to cap Iran’s uranium enrichment at current levels in exchange for a moratorium on further sanctions against it.

A snub by Iran could open a new chapter in the long-running confrontation as President Bush enters his final months in office. While diplomats plan to push for new sanctions, Israel’s leaders and Bush administration hawks, led by Vice President Dick Cheney, argue that military strikes against Iran’s nuclear facilities must be considered.

“It’s clear (Iran) has not complied with the international community’s demand to stop enriching uranium. We, the United States, will work with our allies to come up with another resolution in the Security Council,” Richard Grenell, a spokesman for the U.S. mission to the United Nations, told McClatchy.

The international community offered the “freeze for a freeze” to Iran two weeks ago at a Geneva meeting in which a senior U.S. diplomat, Undersecretary of State William Burns, was present for nuclear talks for the first time.

Israeli Deputy Prime Minister Shaul Mofaz warned Friday that Iran is on the verge of a nuclear breakthrough.

“The window of influence is becoming smaller and, I believe, is about to close,” said Mofaz, a possible candidate to replace outgoing Prime Minister Ehud Olmert.

“Even diplomacy has its limits,” he said at an appearance at the Washington Institute for Near East Policy.