Secretary of State John Kerry pressed ahead on Tuesday in the effort to organize a new round of talks aimed at ending the Syrian civil war that could also align the United States and Russia more closely in strategies for fighting the Islamic State.
The United States and Russia have long been at odds over Syria, with Russia standing firmly by President Bashar al-Assad and the United States backing the rebels seeking his ouster.
On Tuesday, the diplomats said they had narrowed a gap on defining which Syrian militias belonged on a terrorist list.
A final list may be ready Friday, Mr. Kerry said.
Foreign Minister Sergey V. Lavrov of Russia said his country would accept a delegation of Syrian opposition groups at negotiations on Friday, if they arrived under the auspices of a United Nations moderator; Moscow had earlier objected that groups it considers terrorists might wind up in the delegation.