The unidentified speaker sat at the head of a long table with at least 30 other men and a black Islamist flag on the wall.
He listed 14 armed groups as signatories to the statement, including the Al-Nusra Front, Ahrar al-Sham and Liwa al-Tawhid.
The Ahrar al-Sham group rejected the proclamation on its official web page, however, saying that its leadership did not endorse the statement.
The Al-Nusra Front, a formidable fighting force, has claimed the majority of suicide bombings in Syria's deadly 20-month-old conflict.
Abdel Jabbar al-Okaidi, the head of the mainstream rebel Free Syrian Army (FSA) in the embattled northern city of Aleppo, told AFP that the statement did not represent the opinion of all rebel groups in the province.
"These groups represent a number of military factions on the ground and reflect their position, but not all military forces in Aleppo agree with this," the defected former army colonel told AFP by phone.
"The military council has announced its support for the National Coalition and is collaborating with them," Okaidi added.
The new National Coalition aims to present a united front to the international community and is lobbying for weapons supplies to help topple President Bashar al-Assad's regime.
On the ground, fighting flared along the Turkish border after rebels took control of the large army Base 46 in the northern province of Aleppo that had been besieged for weeks.
Six rebels were killed in clashes with Kurdish fighters and the head of the local Kurdish People's Assembly was shot dead in the town of Ras al-Ain, the Britain-based Syrian Observatory for Human Rights reported.
The clashes erupted after a Kurdish demonstration demanding that all rebels not from Ras al-Ain leave after they took the town last week.
The Kurdish fighters belonged to the People's Defence Units, the armed wing of the Democratic Union Party (PYD) which is linked to Turkey's rebel Kurdistan Workers' Party (PKK).
"The rebels burned a flag of the (Kurdish) Democratic Union Party and the Kurds reacted by burning the FSA flag," an activist told AFP.
Rebels accuse Kurdish groups of negotiating directly with Assad's regime, while Kurds question why the rebels entered a safe area.
"The Kurdish regions provide safe havens to thousands of refugees from Damascus, Hama and Homs," PYD leader Saleh Muslim told AFP by phone.
"We are not looking for a confrontation with the FSA, but its members who provoked the incident today in Ras al-Ain receive their orders from Turkey," he said.
Elsewhere, the Observatory said a general it named as Abdullah Darawi was killed in a rebel attack on the town of Nabak, north of Damascus, along with four policemen accompanying him.
The official SANA news agency blamed "terrorists" for the killings.
Fighting also erupted at a border post near the town of Kasab in Latakia province, the Observatory said.
In the mountainous region of Jabal al-Turkman, eight rebels and four soldiers were killed after insurgents attacked an army convoy en route to Kasab.
The Observatory, which relies on a network of activists and medics, said at least 96 people were killed nationwide on Monday -- 45 rebels, 26 civilians and 25 soldiers.
It puts the death toll in more than 20 months of conflict at more than 39,000.