Key Syrian Islamist rebel groups said late Tuesday that they do not recognise any foreign-based opposition group including the National Coalition.
"The National Coalition and the proposed government under (recently chosen) Ahmad Tomeh does not represent us, nor do we recognise it," said 13 of Syria's most powerful Islamist rebel groups.
The groups include members of the main rebel Free Syrian Army and more radical Islamists.
Among the signatories are Liwa al-Tawhid, the main rebel force in the northern province of Aleppo, and the jihadist Al-Nusra Front.
The radical but non-jihadist Ahrar al-Sham also signed on, as did the 19th Division, a significant but relatively new addition to the mainstream FSA.
In their joint statement, they also called for Islamic law to be applied.
"These forces call on all military and civilian groups to unite in a clear Islamic context that... is based on sharia (Islamic) law, making it the sole source of legislation," they said.
They called for "unity" and "to reject division... putting the interest of the (Islamic) nation over the interest of (each) group".
The statement comes amid an escalation of violence pitting fighters from various factions across the rebel spectrum against the Al-Qaeda front group, the Islamic State of Iraq and the Levant (ISIL).