MUNICH — US Secretary of State John Kerry said he was preparing to test Russian seriousness about a Syria diplomatic solution after he and Russian Foreign Minister Sergey Lavrov spoke by phone about possible cease-fire arrangements ahead of an International Syria Support Group (ISSG) meeting here; the Feb. 11 meeting comes after Russian-backed Syrian army advances on Aleppo last week collapsed Syria peace talks in Geneva just days after they had begun.

“What we're doing is testing their [Russian and Iranian] seriousness," Kerry said in an interview with The Washington Post. "And if they're not serious, then there has to be consideration of a Plan B. … You can't just sit there."

The latest high-stakes effort to cobble together common ground on Syria comes as the United States faced growing criticism from European and regional allies about its perceived shirking of leadership on Syria to stem the violence and slow the exodus of millions of refugees that could further destabilize the region and that increasingly risks fracturing European political unity.

"We don't have the feeling that there is a very strong commitment that is there,” French Foreign Minister Laurent Fabius told Reuters, referring to the US role in Syria, as he announced his retirement from the ministry Feb. 10.