"We see the reason that I think eventually Assad will have to be managed out of power," Mattis said on Tuesday. "I don't think any election run under the offices of the Syrian regime is going to have any credibility with the Syrian people or with the international community."
"Russia's best efforts to divert it into Astana process or Sochi have not produced anything worthwhile and so we're calling on Russia to support the UN Geneva process and Staffan de Mistura's efforts there," he continued.
Russia has repeatedly stated that the fate of the incumbent Syrian leader should be decided by the Syrian people exclusively. At the same time, many Western officials, particularly US ones, insisted on Assad's resignation, calling it the main precondition for the Syrian settlement.
In mid-September, German Foreign Minister Heiko Maas announced that the German government could not imagine a long-term political solution with Assad remaining in power. However, Maas did not call the withdrawal of Assad a mandatory prerequisite for Germany's participation in the reconstruction of the country.
Earlier this year, James Mattis said at at a NATO defense minister meeting in Brussels that a US withdrawal from Syria would depend on the success of UN-brokered Geneva peace talks.
Mattis also claimed that a US withdrawal from Syria would create a "vacuum" that President Bashar Assad and his allies would "take advantage of".