BEIRUT — Syrian activists say that a wave of tit-for-tat kidnappings between rival Islamic militant groups in the northern city of Aleppo risks sparking large-scale internal fighting between rebels.

Rami Abdul-Rahman, who heads the Britain-based Syrian Observatory for Human Rights, said Saturday that a coalition known as the Judicial Council had accused another rebel force, the Ghurabaa al-Sham, of robbing factories in Aleppo's industrial neighborhood.

He said the two groups clashed on Tuesday and the Judicial Council is now holding Ghurabaa al-Sham members captive. The Ghurabaa al-Sham is also holding Judicial Council members.

Aleppo-based activist Mohammed Saeed said Ghurabaa al-Sham withdrew its fighters from several neighborhoods, including the industrial area.

Aleppo is Syria's largest city and is split between rebel and government control.

Associated Press writers Vladimir Isachenkov in Moscow and Matthew Lee in Washington contributed to this report.