Activists in Aleppo reported tense anticipation after the group retook parts of the city where its ejection had been celebrated days earlier and where civilian activists and citizen journalists — who had faced arrests and killings from the group — had come back out into the open.

In the Aleppo neighborhood of Al Bab, Islamic State of Iraq and Syria fighters established checkpoints, demanded identification cards and examined cellphones in a search for “suspects who are operating or collaborating with the rebels,” said Mohammed Wissam, an activist with the antigovernment Aleppo Media Center. Such activities have contributed to a belief by many opponents of the Islamic State of Iraq and Syria that the group is serving the government.

“ISIS took over the area earlier today after rebels pulled out,” Mr. Wissam said. “ISIS told the residents that they came to fight the corrupt insurgents and apply God’s law, calling on everyone to cooperate.”

While the group’s fighters warned that any attack on them would bring “brutal retaliation,” he said, there had been no documented arrests or killings. “A lot of people are fleeing the area,” he said.

During the infighting, government forces retook part of an important Aleppo industrial neighborhood, the Aleppo Media Center said, and if they are successful in reclaiming the remainder, “the whole liberated area of Aleppo would be isolated and besieged.”

Separately, the Foreign Ministry in Italy said that the Italian port chosen for the transfer of Syrian chemical weapons to an American naval vessel equipped to neutralize them at sea would be announced on Thursday. In a statement, the ministry said the announcement would be made by Ahmet Uzumcu, the director general of the Organization for the Prohibition of Chemical Weapons, the Hague-based group that is collaborating with the United Nations to oversee the destruction of Syria’s chemical munitions.

That collaboration, which began in early October after Syria’s government agreed to renounce chemical weapons and join the treaty that bans them, has resulted in a high level of assistance from other governments, regardless of their views on the Syrian conflict. China and Russia, for example, are providing security for the export of the most dangerous chemicals, and a Danish vessel will be transporting them to Italy from Syria.

Germany agreed last week to help destroy waste from the chemicals to be neutralized by the American vessel, the Cape Ray. Britain’s foreign secretary, William Hague, announced on Monday that his country would provide additional equipment to the Cape Ray to help speed the process for rendering the chemicals harmless.