BEIRUT — Seven Syrian Islamist rebel groups announced Friday that they’d combine operations in the face of a fierce offensive by troops loyal to President Bashar Assad, a move that would turn the disparate groups into the largest anti-Assad faction.

The groups said their new affiliation would be called the Islamic Front and would aim to replace Assad’s government with an Islamic state.

Two of the anti-Assad movement’s most effective forces, the Nusra Front and the Islamic State of Iraq and Syria, both of which are al-Qaida affiliates, weren’t named as part of the new coalition. But the new grouping left the door open to cooperating with them.

“The Islamic Front is an independent military and social force that is aimed at bringing down Assad’s regime in Syria and at replacing it with a just Islamic state,” the groups said in a statement.

The Islamic Front would be composed of the largest rebel fighting force in the north, Liwa Tawhid, and the most powerful faction fighting in Damascus, Jaysh Islam. The other groups that agreed to the new unit are Ahrar al-Sham, Liwa al-Haq, Ansar al-Sham and Suqour al-Sham, as well as a smaller Kurdish group, the Kurdish Islamic Front.

In an interview with the Al-Jazeera satellite news channel, an official from the new group said the negotiations to form the new command had taken seven months.

Amad Essa al-Sheikh, the head of the Consultative Council of the new Islamic Front, said the goal of integrating the factions was to bring about “a paradigm shift in the armed rebellion by closing ranks and mobilizing them to become the real alternative to the dying regime.”

It was the second major reorganization of the rebels, and it seemed to cement the end of the role of the U.S.-backed Supreme Military Council as the primary coordinator of rebel factions. In September, some of the same groups announced that they’d no longer recognize the authority of either the Supreme Military Council or the Syrian Opposition Coalition, the civilian umbrella group that the United States and other Western countries have recognized as the legitimate Assad opposition.