Seven leading Islamist rebel groups in Syria say they are joining forces.
A statement posted online said Ahrar al-Sham, Jaysh al-Islam, Suqour al-Sham, Liwa al-Tawhid, Liwa al-Haqq, Ansar al-Sham and the Kurdish Islamic Front had agreed to a "gradual merger".
It said the new Islamic Front will be an "independent political, military and social formation" to topple the Assad regime and build an Islamic state.
The move comes as government forces make advances on key rebel-held areas.
In the past month, several towns around the capital Damascus and the second city of Aleppo have been recaptured by soldiers backed by pro-government militiamen, members of the Lebanese Shia Islamist movement Hezbollah and Iranian Revolutionary Guards, activists say.
An offensive has also been launched in the Qalamoun mountains, prompting thousands of refugees to flee into nearby Lebanon.