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As President Obama considers the ground rules he can live with for a resumption of talks with Iran on its nuclear program, a wary Congress is telling the White House: Accept no compromises.

Specifically, a bipartisan group of influential senators is insisting that Iran be required to verifiably suspend all of its uranium enrichment activity during any negotiations 鈥� even though such a demand could sink prospects for a new round of talks.

In a letter addressed to Mr. Obama, a dozen senators representing both major political parties and a range of red and blue states say they 鈥渟trongly oppose any proposal that caps or limits sanctions against the Iranian regime in exchange for anything less than full, verifiable, and sustained suspension of all enrichment activities, including both 3 percent and 20 percent enrichment.鈥�

Uranium enriched to 3 percent purity is used to fuel nuclear power plants, but Iran has also been enriching to 20-percent purity, setting off alarm bells among world powers. They worry Iran is stockpiling enough 20-percent-enriched uranium to make the 鈥渂reakout鈥� leap to the 90 percent highly enriched uranium it would need for a nuclear weapon.

Iran counters that its nuclear program is for purely peaceful purposes, and that it is using the 20 percent enriched uranium to operate an aging, American-built research reactor near Tehran that makes isotopes for medical use.

The letter from the senators 鈥� led by Democrat Bob Menendez of New Jersey, Republican Jim Risch of Idaho, and Independent Joe Lieberman of Connecticut 鈥� arose after Secretary of State Hillary Rodham Clinton and the European Union鈥檚 foreign policy chief, Catherine Ashton, suggested Friday that talks between Iran and the so-called P5 +1 countries could resume soon. The P5 + 1 group includes the five permanent members of the United Nations Security Council plus Germany.

Iran last week sent a letter to Lady Ashton saying it was ready to resume talks on its nuclear program. But Iranian officials have also insisted that they will never agree to suspend all enrichment in exchange for talks.

The senators鈥� tough stance reflects growing concerns among foreign policy hawks that the Obama administration could soften its position on Iranian enrichment in an effort to relaunch talks that were last held 鈥� and which got nowhere 鈥� more than a year ago.

Administration critics see a redrawing of the administration鈥檚 red lines on Iranian enrichment: They note that in 2009 Secretary Clinton flatly rejected Iran maintaining any enrichment capabilities, while in recent statements her position has become more nuanced.

A recent formula offered by Clinton speaks of guaranteeing the peaceful nature of Iran鈥檚 nuclear program while respecting Iran鈥檚 right to the peaceful use of nuclear technology 鈥渃onsistent with its obligations under the Non-Proliferation Treaty.鈥�

Some senators fear that position would leave the door open to Iran maintaining some level of enrichment activity while talks took place.

Mounting pressure on the Obama administration reflects rising tensions over Iran鈥檚 nuclear program. On Sunday Iran announced it was cutting off oil shipments to France and the United Kingdom, two EU and permanent Security Council members who have been among the most critical of Iran鈥檚 nuclear program.

Last month the EU approved an embargo on oil imports from Iran, to take effect in July.

Obama is scheduled to meet at the White House March 5 with Israeli Prime Minister Benjamin Netanyahu, who will be in Washington to address the pro-Israel lobby AIPAC, the American Israel Public Affairs Committee.

The administration has been working to convince Israel to give the toughened sanctions the international community has imposed on Iran a chance to have an impact, and at the same time seeking to compel Iran to backtrack on its nuclear activities.

In their letter, the senators say they are 鈥渆xtremely concerned鈥� that Iran will try to use any negotiations to 鈥渂uy time鈥� to make further progress in its nuclear activities. They also warn against any attempt Iran might make to use talks to 鈥渄ilute 鈥� the current momentum of the pressure track鈥� whereby sanctions are placing increased constraints on the Iranian economy.

Also signing the letter to Obama were Senators John McCain聽(R) of Arizona, Richard Blumenthal (D) of Connecticut, Kelly Ayotte (R) of New Hampshire, Kirsten Gillibrand (D) of New York, Marco Rubio (R) of Florida, Ben Cardin (D) of Maryland, Pat聽Toomey (D) of Pennsylvania, Chuck Schumer (D) of New York, and Lindsey Graham (R) of South Carolina.