VIENNA — A yearlong effort to seal a nuclear deal with Iran fizzled Monday, leaving the United States and its allies little choice but to declare a seven-month extension in hopes that a new deadline will be enough to achieve what a decade of negotiations have failed to do – limit Tehran’s ability to make a nuclear weapon.

Secretary of State John Kerry and other Western foreign ministers converging on Vienna in a last-ditch diplomatic push talked up the add-on time as the best way forward. “We would be fools to walk away,” Kerry said.

But a week of tough maneuvering appeared to have achieved little more than agreement to keep on talking. Negotiators will now strive to nail down by March 1 what Iran and the six world powers it is negotiating with must do, and by when. A final agreement is meant to follow four months later.

Pushback from critics in Congress was not long in coming, reflecting the concern of powerful Republicans that Iran is merely trying to buy time – criticism that is likely to increase if negotiations yield little progress in capping Iranian programs that could make nuclear weapons.

Members of the new Republican-controlled Congress to be sworn in early next year have already threatened to impose additional sanctions on Iran and may well have enough votes to overturn an expected veto by President Obama.

“The one thing the Iranians didn’t have was time, and now they have 219 days,” lamented Sen. Mark Kirk, an Illinois Republican whose work with Democratic Sen. Bob Menendez of New Jersey on oil sanctions helped cripple Iran’s economy and drive it to the negotiating table.

Kirk pledged to come forward with a new bipartisan sanctions package after the Republican takeover of the Senate. Menendez suggested similar action, saying he’d work “to ensure that Iran comprehends that we will not ever permit it to become a threshold nuclear state.”

The U.S. administration strongly opposes additional sanctions because it fears it will push Tehran away from the table.

Kerry called for patience, saying he hoped congressional skeptics would “come to see the wisdom” of giving talks an extra “few months to be able to proceed without sending messages that might be misinterpreted.”

In Tehran, hard-liners fearful that their country will give away more than it gets under any final deal may increase pressure on Supreme Leader Ayatollah Ali Khamenei to break off talks. Still, the latest extension appears to have the approval of Khamenei, the ultimate arbiter in his country.