WASHINGTON — The United States on Monday suspended talks with Russia over the protracted conflict in Syria, accusing the Kremlin of joining with the Syrian Air Force in carrying out a brutal bombing campaign against the besieged city of Aleppo.

Anticipating the end of the talks after repeated warnings from American officials, President Vladimir V. Putin of Russia responded by withdrawing from a landmark arms control agreement that calls for each side to dispose of 34 tons of plutonium, a material used in nuclear weapons.

The developments signaled the further deterioration of relations between the United States and Russia, which are now bitterly at odds over Syria, Ukraine and other issues.

“Cooperation over Syria was the Obama administration’s last and best shot for arresting the downward spiral in the bilateral relationship with Russia,” said Andrew S. Weiss, a former White House expert on Russia who is vice president for studies at the Carnegie Endowment for International Peace. “The mistrust and hostility toward the United States by the Russian leadership is real and growing. It is going to be the driving force behind Russian external behavior for many years to come.”