A source at Abdel-Halim Khaddam's office said the former official held talks with Ali Bayanouni, head of the Sunni Islamist group, in Brussels on Tuesday and Wednesday.
The source told Reuters in Beirut by telephone: "There was agreement on a joint vision to save Syria from the crisis that the regime has placed it in. It was also agreed to contact other opposition leaders inside and outside Syria to come up with a joint plan of action."
The source said the two leaders also rejected any foreign intervention in Syria: "The responsibility of changing the corrupt regime in Syria lies only on the Syrian people."
Bayanouni confirmed the meetings in a telephone interview with the Arabic television station Aljazeera.
He said: "The most important aspect we agreed upon in our last meeting is the need to work with other national powers and hold meetings with them ... to agree a formula for co-operation amongst us."
For 30 years, Khaddam was a political ally of Hafez al-Assad, Syria's former president and Bashar al-Assad's late father.
Now he is promoting a detailed "national action programme" that calls for a transitional government in Syria to take the country from the totalitarian regime to a democratically elected parliament that appoints a government.
It also calls for a separation between the powers of president and those of the legislative and executive bodies, the abolishing of a four-decade state of emergency and the release of all political prisoners.
Al-Assad is facing growing
The programme also stipulates the democratic government would commit to the return of the Golan Heights, occupied by Israel since 1967, in any peace deal with the Jewish state and respect all UN Security Council resolutions.
Khaddam, a Sunni Muslim who quit in June and has since moved to Paris, broke away from al-Assad in December.
He said last month that al-Assad was facing growing pressure from economic problems at home and the international investigation into the killing of Rafiq al-Hariri, a former Lebanese prime minister.
Syria, led by al-Assad since his father died in 2000, faces international isolation and possible UN sanctions unless it co-operates fully with the inquiry into the murder of al-Hariri a year ago.
The inquiry has implicated Syrian officers in the killing and Khaddam has said al-Assad personally had threatened al-Hariri and that the assassination could not have taken place without direct orders from the president.
Damascus has denied any links to the murder and has not agreed to let investigators question al-Assad, though other security officials have been quizzed in Vienna.
The Brotherhood, founded in Syria in 1945, is widely seen as the most serious rival to the Baath Party which in 1980 made membership of the group punishable by death.
It became involved in violent opposition to Hafez al-Assad's military-backed government in which his fellow minority Alawites held many key posts, culminating in an uprising that was ferociously suppressed in the town of Hama in 1982, where many thousands died.
More than 70% of Syria's 18 million population are Sunni Muslims.