Rafif Jouejati, an American of Syrian ancestry who is a spokeswoman for a network of activists in Syria, said those committed to Mr. Assad’s removal had no interest in “a foreign transition plan,” however well intentioned.

“What we don’t want to do is descend into the total chaos that Iraq did,” said Ms. Jouejati, who is participating in a similar planning effort among Syrian activists coordinated through the United States Institute of Peace, an independent but Congressionally financed organization in Washington. Even so, she added, “I don’t think we want the United States to impose lessons learned here.”

The State Department and Pentagon planning efforts became more systematic last month after hopes for an internationally brokered resolution faltered in the face of Russian and Chinese opposition in the United Nations Security Council. The planning is being closely coordinated with regional allies like Turkey, Jordan and Israel, and it coincides with an expansion of overt and covert American and foreign assistance to Syria’s increasingly potent rebel fighters.

While the administration has ruled out arming the rebels directly, the administration has authorized $25 million in direct assistance for medical supplies and communication equipment to help the fighters and civilian opponents of Mr. Assad coordinate their activities and, crucially, disseminate reports about the fighting to the rest of the world.

Other countries, including Turkey, Saudi Arabia and Qatar, are providing weapons, assisted by a small number of officers from the Central Intelligence Agency who are vetting the fighters receiving them and working with State Department officials trying to unify the fighters with political leaders inside and outside the country. Last month, the Treasury Department granted a waiver to let a new American organization, the Syrian Support Group, raise money for the rebels despite the sanctions that prohibit most financial transactions in Syria.

On Thursday, President Obama also announced a $12 million increase in humanitarian aid, bringing the total to $76 million, largely distributed through international organizations like the World Food Program.

The State Department effort is being coordinated by Deputy Secretary of State William J. Burns, who worked in the Near Eastern Affairs Bureau during the planning for the American invasion of Iraq in 2003, when the department clashed with the Pentagon over what to do after Mr. Hussein’s fall. The department has created a number of separate cells devoted to aspects of a post-Assad Syria, including humanitarian issues, economic reconstruction, security, the stockpiles of chemical weapons and a political transition.