George Sabra, a spokesman for the SNC, told a news conference in İstanbul that the broad-based opposition group had decided to arm the Free Syrian Army and added that some foreign governments were helping to send weapons.
"We demand military intervention by Arab and Western countries to protect civilians," Sabra said, speaking a day before SNC representatives were due to meet UN-Arab League envoy Kofi Annan in Ankara.
"We demand establishment of secured humanitarian corridors and zones to protect the civilians. We demand implementation of a no-fly zone over entire Syria to prevent Assad from continuing massacres."
He said the SNC had established a coordinating bureau to channel arms to the Free Syrian Army, with the help of foreign governments, but he declined to say where the bureau was located or which governments were involved.
"The Syrian National Council has taken concrete and practical decisions to arm Free Syrian Army that is established to protect the civilians. And we invite all colonels and other military officials in the Syrian army to take sides with the people of Syria," Sabra said.
Annan, who held talks with Assad in Damascus over the weekend, was due to meet the opposition representatives at 0800 GMT in Ankara on Tuesday, and to make a statement afterwards.
He met Turkey's Prime Minister Recep Tayyip Erdoğan and Foreign Minister Ahmet Davutoğlu to discuss the crisis, having first gone to Doha from Syria, as Qatar chairs the Arab League.
Annan said he was trying to get everybody to the table through a political process, while ensuring that humanitarian access is opened and the killing of civilians stops.
"Since I came to the region I am following very, very closely developments in Syria and there are grave and appalling reports of atrocities and abuses," he told reporters in Ankara.
"Killings of civilians must end now. The world must send a clear and united message that this is simply unacceptable."
Ankara has turned against its former friend Assad.
Aside from providing refuge for more than 12,500 Syrians who have fled their homeland over the past year, the Turkish government has allowed the opposition to meet in İstanbul, and provided sanctuary for opposition fighters in the Free Syrian Army.