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The warning followed similar threats Tuesday by Israeli Prime Minister Benjamin Netanyahu, who asserted, after meeting with his security advisers, that Israel “is ready for any scenario.”

“We are not part of the civil war in Syria,” Mr. Netanyahu said. “But if we identify any attempt whatsoever to harm us, we will respond in strength.”

The specter of such high-stakes posturing appeared to hang Thursday over the Obama administration, which has spent much of this week trying to assure the American public and the international community of the limited scope of a possible strike.

“Nobody’s talking about regime change through military options in Syria,” Ms. Harf told reporters at the State Department on Thursday.

Meanwhile, some foreign policy insiders in Washington say that privately, Israel is actually wary of inflaming tensions with Iran at a time when Tehran appears to be on the verge of caving to Western sanctions aimed at containing its nuclear program.

Iranian President Hassan Rouhani may be more likely than his predecessor, Mahmoud Ahmadinejad, to pursue positive nuclear negotiations with the West, where fears have mounted in recent years that Tehran is close to developing a warhead. Iran claims its nuclear program is peaceful.

Israel does not want to be viewed as jamming the international negotiations with a new Iranian leader who claims to be pragmatic,” said David Makovsky, a scholar at the Washington Institute for Near East Policy. “It’s hard for me to see Israel taking a step that is bound to create a lot of international contentiousness at this juncture.”

At the same time, Mr. Makovsky said: “The U.S. has telegraphed very publicly that it doesn’t see these strikes [on Syria] as a way of trying to bring the U.S. into the war and, therefore, its very possible the Iranians just ride it out.”