The Obama-Putin meeting was held as Moscow played host to an international negotiating session with Iran. Russia has gone along with U.N. Security Council efforts to tighten some penalties against Iran because of questions about its nuclear weapons ambitions, but has blocked the harshest punishments.

The meeting was one of two major foreign policy challenges preoccupying Obama during his two days at the Group of 20 economic meeting. Much of the rest of the summit was to be devoted to the European fiscal crisis and the fate of Greece as a part of the euro zone. A pro-euro candidate is trying to form a Greek coalition government following elections Sunday, but the anti-austerity second-place party has refused.

The G-20 gathering is a natural forum for sideline discussions of the urgent crisis in Syria as well as diplomatic efforts to head off a confrontation with Iran. Russia is a linchpin in world efforts to resolve both crises, and to U.S.