ISTANBUL — Turkey and the United States agreed Saturday to accelerate preparations for the possible fall of Syria’s president, Bashar al-Assad, creating a formal bilateral team to manage helping the opposition, providing aid to fleeing refugees and planning for worst-case outcomes that include a chemical weapons attack.

At a news conference here, Secretary of State Hillary Rodham Clinton and Turkey’s foreign minister, Ahmet Davutoglu, said that with the situation in Syria growing more dire — as the battle for Aleppo continues to rage — it was time to create a nerve center for information sharing and planning. They said a unified task force with intelligence, military and political leaders from both countries would be formed immediately to track Syria’s present and plan for its future.

“What the minister and I agreed to was to have very intensive operational planning,” Mrs. Clinton said. “We have been closely coordinating over the course of this conflict, but now we need to get into the real details.”

Mrs. Clinton, who also announced an additional $5.5 million in humanitarian assistance for refugees, left open the possibility of setting up a no-fly zone, suggesting that the new planning team assigned to perform an “intense analysis” of all options could be a precursor to more direct assistance. But she stopped short of describing specific plans for helping Syria’s opposition fighters now, or the timing.