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A second U.S. diplomat noted that, unlike Libya, Syria has no rebel-held cities and few high-ranking army defections. “The opposition is not going be able to overthrow the government by force,” he said. “This is not a repeat of Libya.”

On Tuesday, Iran’s semiofficial Fars News Agency claimed that Mr. Assad, during a meeting with Turkey’s Foreign Minister Ahmet Davutoglu had threatened a regional war in the event of Western military intervention.

Mr. Assad said Syria and Hezbollah in Lebanon would launch hundreds of rockets into Israel and Iran would attack U.S. warships in the Persian Gulf, according to the report.

But a senior Turkish official told The Times that the Iranian news report was “completely untrue.”

Ben Birnbaum contributed to this report.