BEIRUT, Lebanon — President Bashar al-Assad of Syria called on insurgents fighting his government to join with his forces instead, and do battle against the Islamic State. Only after that group is defeated can there be a political solution to the war that has devastated the country, Mr. Assad said in an interview broadcast on Wednesday.

Mr. Assad spoke to Russian state news media outlets at a time when Russia is stepping up its military aid to his government and its diplomatic efforts to break the bloody deadlock in Syria. Moscow has been urging Washington and its allies to set aside their calls for Mr. Assad’s ouster and work with him against the Islamic State.

The United States has sought to cooperate only with more moderate insurgents, not with the Syrian government, in fighting the extremist group.

Russia’s recent moves suggest that it is trying to position itself as the white knight in the Syrian conflict.