BERLIN — Chancellor Merkel said Germany will not back a military strike on Iran, underlining the difficulty that America faces in keeping international support for its effort to contain Iranian nuclear ambitions.

Iran, which last week defied a U.N. demand that it halt uranium enrichment, said yesterday that it is ready to retaliate against any attack. Negotiators from the five permanent members of the U.N. Security Council and Germany will meet in Berlin today to discuss whether to impose sanctions.

“The military option isn’t an option,” Mrs. Merkel told Parliament yesterday, adding to comments she made September 2. “We won’t close the door to negotiations.”

Germany is the first country involved in negotiations with Iran to rule out the use of force and Mrs. Merkel’s pledge divides her from President Bush, who has repeatedly refused to do so.

Russia may be willing to support economic penalties on Iran, Foreign Minister Sergei Lavrov said yesterday, according to a report by Agence France-Presse. France and Britain have said Iran must stop enriching uranium before they will resume negotiations.

President Ahmadinejad of Iran has vowed not to back down, betting that America and its partners in the nuclear talks will not be able to maintain a united approach. Iran has said it is enriching uranium to fuel nuclear power generators that are being built.

Negotiators will probably discuss putting travel restrictions on Iranian citizens and diplomats, freezing the nation’s offshore assets, and limiting trade in technical products during their meeting in Berlin tomorrow.

“They’re trying to make life a little harder for the Iranians,” an analyst at the Cato Institute in Washington, Justin Logan, said in an interview last week. “We’re unlikely to see any petroleum embargo right now.” Iran holds the world’s second-largest oil reserves.

Iran successfully test-fired “a variety” of laser bombs as part of a military exercise, state-run Fars News reported yesterday. Iran’s defense industry can manufacture warplanes, and the military has other “tools” that will surprise its enemies, Iranian Major General Ataollah Salehi said, according to the news agency.

President Bush on Tuesday sought to link the Iranian leadership with the goals of Al Qaeda and compared Iran with a “regime of tyranny.” He also said in a speech in Washington that Iran would not be allowed to build nuclear weapons.

The Iranian president yesterday dismissed those comments, Agence France-Presse reported.

“I am telling him that all the world is threatening you since the general path that the world is taking is toward worshipping God and divinity,” Mr. Ahmadinejad said yesterday at a news conference in Tehran, according to AFP.