BEIRUT -- Syrian aid workers said Friday that they have suspended their work inside Aleppo, Syria’s largest city, in anticipation of a bruising battle between rebels and forces loyal to President Bashar Assad in what would certainly be a climactic moment, if not the climax, in an increasingly complicated civil war.

Preparations for a major battle for Aleppo came as two Western journalists who’d been missing for a week reported that they’d been held by a group of foreigners who told them they planned to bring Islamic law to Syria. It was the first confirmation of long-held suspicions that Islamist extremists were operating inside the country.

With warring sides reportedly pouring men and material into Aleppo, a city that had once been Syria’s economic hub and which for the past 17 months has been a refuge for those fleeing violence elsewhere, the Syrian Arab Red Crescent, the country’s Red Cross, said it had become too dangerous for its workers to continue providing assistance to the thousands of displaced people who thought they’d found a relatively peaceful haven from the violence.

Rabab al Rifai, the Damascus spokeswoman for the International Committee of the Red Cross and Red Crescent, said in a telephone interview that the suspension came “because of the very tense and highly volatile situation” in the city and the surrounding countryside.

Other aid workers said they were busy converting schools into shelters for the tens of thousands of people who are expected to or have already fled their homes in the week since rebel forces took up positions in what had been calm neighborhoods. Residents said they believe a brutal government counterattack to retake control is all but certain.

Nael Hariri, a physician on the city’s north side who’s been assisting evacuees, said government shelling of some rebel-held areas already has caused massive casualties and destruction. “There are many buildings that have already been brought down,” he said in an interview conducted over Skype.

“The FSA is responsible in some ways, but the regime is not helping,” he said, using the initials for the rebel Free Syrian Army. “It’s not a solution to destroy what’s left of Syria.”

Just how much of Aleppo, a city of 2.1 million that covers 85 square miles, is under rebel control was an open question. On Friday, rebels posted a video on YouTube purportedly showing dozens of soldiers and pro-government militiamen captured by rebels in Furqan, a neighborhood on Aleppo’s west side. A rebel sympathizer, reached on Skype, compared the rebel presence in the city to Benghazi, the city in eastern Libya that fell under rebel control early in that country’s uprising against Moammar Gadhafi.

The comparison seemed optimistic at this point, however, with others reporting a far more chaotic and less certain rebel presence.

Hariri, the physician, described the rebels as acting “spontaneously, with no clear plan,” as they popped up in neighborhoods across the city.

Just who the rebels are and how they had managed to infiltrate a city that had been under tight government control until only a few days ago also were unanswered questions.

In describing themselves in a video posted on the Internet, the rebels used the term “tawhid” for their group, a word that in Arabic means “unity” or “monotheism” and is generally associated with conservative Islamists known as Salafis, who are active throughout the Middle East and Central Asia. That word has not generally been used in other videos posted by groups describing themselves as units of the Free Syrian Army.