Syrian dictator Bashar Assad has “hamstrung” attempts to negotiate an end to the civil war that has racked his country since 2011, a top U.S. diplomat says.

Special envoy James Jeffrey has been working with the United Nations to negotiate a political settlement to end the Syrian civil war. That effort has been complicated by international jockeying for leverage, as Russia and Iran intervened with airpower and ground forces to stabilize Assad’s regime. But Jeffrey allowed that Russia has been “helpful” in the talks, putting blame for the stalemate on the Syrian dictator. “The Russians have been fairly, I would say, forthcoming in ideas and other things,” Jeffrey told reporters this week. “I think the problem is that the Russians are hamstrung by the Syrian regime, which doesn’t want anything to do with this.”