ISTANBUL — Turkey and the United States agreed Saturday to accelerate preparations for the possible fall of Syria's president, Bashar Assad, creating a formal 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," Clinton said. "We have been closely coordinating over the course of this conflict, but now we need to get into the real details."

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 at some point, noting that all options under consideration require "intense analysis."

Hinting at fears of a wider war, Clinton said Saturday that the goal was to hasten the removal of Assad but "not in a way that produces even more death, injury and destruction."

Turkey is a natural hub for any kind of action in Syria. A former Syrian ally, it declared its allegiance with the rebels; many Syrian opposition groups are based in Turkey, and its Syrian border has become the main distribution point for weapons and assistance to the rebels, who have opened an on-again, off-again supply corridor from the border to Aleppo.

On Saturday, Davutoglu spoke more forcefully than Clinton on the need for action. Describing the humanitarian crisis in Aleppo and the plight of refugees fleeing on roads under attack from Syrian forces, he said, "The international community needs to take some very decisive steps to stop this."

But in practice, analysts said the U.S. and Turkey, along with a wider group of allies known as the Friends of Syria, continue to hold back. Saturday's announcement still amounts to a policy of life support, some argue, giving enough help to keep the rebels alive and minimizing intervention while figuring out what to do next.

"The friends of Syria have developed a stake in making sure the opposition is simply not wiped out," said Ilter Turan, an international-relations professor at Bilgi University in Istanbul. "That becomes the ruler to measure this by."

After 17 months of conflict and at least five months of the U.S. focus on "nonlethal assistance," some signs of international help have, in fact, recently been seen. More rebel commanders in Syria have satellite phones and ways to mask their communications.

Rebels and activists say such assistance so far has been nowhere near enough, in quantity or substance.

Arab foreign ministers plan to meet on Sunday in Jeddah, Saudi Arabia, to discuss developments in Syria and to consider selecting a replacement for Kofi Annan, the United Nations-Arab League envoy who resigned this month.

Rebels said that there was fierce fighting Saturday in Aleppo and Damascus. Activists also told Reuters that Syrian and Jordanian troops had clashed near the border as refugees tried to cross, reflecting rising tensions in the area where Syria's prime minister recently defected.

And despite pleas for foreign help, the rebels said they would find their own ways to survive and advance. On Saturday, fighters from the main brigade in Aleppo posted a video purportedly showing the successful seizure of a government weapons arsenal. Dozens of automatic weapons stood in piles alongside steel green boxes of ammunition as rebels could be heard declaring, "God is great."