Lavrov also suggested Biden’s statement that Assad must go was counterproductive.
‘‘The persistence of those who say that priority No. 1 is the removal of President Assad — I think it’s the single biggest reason for the continued tragedy in Syria.’’
Asked whether Russia might be prepared to endorse humanitarian corridors protected by air power, Lavrov bluntly replied: ‘‘No. Any threat of use of force will be unacceptable.’’
Jim Heintz contributed from Moscow.