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In Moscow, a Kremlin spokesman said Mr. Obama, in his phone call to Mr. Putin, failed to persuade the Russian leader to endorse tougher sanctions against Mr. Assad. “Differences in approaches remain,” he said.

In Washington, the State Department raised concerns that the security apparatus surrounding Mr. Assad is beginning to falter.

“We think the regime is losing control in Syria,” said State Department spokesman Patrick Ventrell, who added that U.S. officials are “watching the situation very closely” amid concerns that a chaotic power vacuum could open if Mr. Assad is assassinated.

Unease over the development was palpable at the Pentagon, where Mr. Panetta told reporters that Syria’s security situation is “spinning out of control.” He was joined by British Defense Minister Philip Hammond, who added that “there is a sense that the situation is deteriorating and is becoming more and more unpredictable.”

Both men stressed the need for the international community to unite and to put collective pressure on the Syrian government to persuade Mr. Assad to step down. U.S. officials hope such a development could open the window for a peaceful political solution to the mounting violence between the Syrian military and opposition forces.

The Treasury Department on Wednesday announced measures to freeze the assets of 29 senior Syrian government officials, but past attempts to impose broader sanctions have been weighed down by diplomatic resistance from Russia, which maintains a naval base in Syria.

U.S. officials lobbied the U.N. Security Council on Wednesday to enact a resolution allowing for wider sanctions against the Assad government. Russia has threatened to veto such a resolution.

• Mr. Resneck reported from Hasankeyf, Turkey, while Louise Osborne in Berlin and Guy Taylor in Washington contributed to this article, which is based in part on wire service reports.