A Senate bill intended to tighten economic sanctions against Iran was blocked Thursday by Senate Republicans who said they were holding out for an even tougher measure that included an explicit threat of a U.S. military strike if Iran refuses to curb its nuclear ambitions.

The surprise opposition to the sanctions bill — which was endorsed by a Senate committee two months ago with bipartisan support — left in doubt the fate of Democrat-backed legislation that sought to close loopholes and crack down on international banks that help Iran sell its oil to overseas customers.

Republican resistance to the measure visibly irked Senate Majority Leader Harry M. Reid (D-Nev.), who accused GOP leaders of trying to score political points by raising last-minute objections to a bill that has been in the works since March. Republicans claimed they had not had enough time to study the bill.

“This has been a classic example of rope-a-dope,” Reid complained in remarks to the Senate chamber after the measure was put on hold. “Two months ago, I came to the Senate floor and said we need to pass the sanctions immediately. . . . I have been working the last two months trying to get this done.”

Sen. Lindsey O. Graham (R-S.C.), a senior member of the Senate Armed Services committee, told the Senate that the bill did not go far enough in warning Iran of the consequences of its alleged march toward nuclear-weapons capability. Iran has consistently said its program is entirely peaceful. Nuclear talks between Iran and six world powers are set to begin Wednesday in Baghdad.

“I don’t want to create a document before negotiations . . . that doesn’t include something beyond sanctions to change the Iranian behavior we all want to avoid,” Graham said.

Graham said he wanted to add wording to “send an appropriate signal” to Iran that a U.S. military strike would be seriously considered if Iran failed to change its behavior.

President Obama has repeatedly warned Iran that “all options are on the table” — including military action — to block its efforts to make nuclear weapons. But the White House has endorsed a strategy of political and economic pressure as the preferred means of forcing Iran to halt its pursuit of weapons-sensitive technology.

The Senate legislation would have built on economic sanctions approved by Congress last year to punish banks and businesses that help Iran sell its oil abroad. The proposed measure, known as the Johnson-Shelby bill, also would have imposed penalties on companies or governments that help Iran block Western radio and Internet transmissions or provide it with tear gas and other weapons used against opposition groups.

The congressional debate came amid disclosures of new U.S. assurances to Israel that Washington was prepared to use force against Iran if diplomacy failed. U.S. Ambassador to Israel Dan Shapiro gave the assurances to Israeli leaders on Tuesday in what was billed as a private meeting with Israeli bar association members. A copy of his remarks was shared with news organizations Thursday.

“It would be preferable to resolve this diplomatically and through the use of pressure than to use military force,” Shapiro was quoted as saying. “But that doesn’t mean that option is not fully available. And not just available, but it’s ready.”

State Department officials on Thursday defended Shapiro’s remarks as in line with previous statements by Defense Secretary Leon E. Panetta and other senior administration officials.