In a potential sign that Russia’s support of embattled Syrian President Bashar al-Assad may be softening, Moscow’s top diplomat will meet jointly Thursday with Secretary of State Hillary Rodham Clinton and the U.N. envoy for Syria, a senior State Department official said.

Russia has been the main international defender of the Assad regime, a military and trade partner, and the chief obstacle to tougher U.N. action to pressure Assad to end a 20-month civil war. …

The State Department official gave no exact agenda for the session among Clinton, Russian Foreign Minister Sergei Lavrov and U.N.-Arab League envoy Lakhdar Brahimi. The official requested anonymity in talking about the meeting, because the hastily arranged three-way session will be private.

But the goal for the United States was clear. A stronger U.N. mandate would put further pressure on Assad to step down, and Russian agreement with such an effort would leave Assad with no meaningful international support.