Gen. Salim Idris, who defected from the Syrian army in July, told The Associated Press in an interview the rebels could defeat the regime within a month if supplied with anti-aircraft weapons. Without foreign military help, he estimated it could take up to three months.
Assad's troops are stretched thin and have lost ground in recent months, particularly in northwestern Syria, but have kept rebel fighters pinned down with massive air bombardments. Idris claimed that more than 120,000 armed men are fighting Assad's military, a figure difficult to confirm independently in the chaos of the civil war.
Idris said the rebels are trying to monitor the regime's chemical weapons sites.
Syria is said to have one of the world's largest chemical arsenals. Earlier this week, Syria's U.N. ambassador said the regime would not use such weapons under any circumstances. However, recent U.S. intelligence reports indicated the regime may be readying chemical weapons and could be desperate enough to use them.
The regime "can and will" use chemical weapons unless the international community forces Assad to leave, Idris said. "We know exactly where they are and we are watching everything," Idris said. "But we don't have the capability to put them under our control."
The West has shown little desire to intervene militarily in Syria's conflict, but President Barack Obama has said the regime's use of chemical weapons against the rebels would be a "red line."
Earlier this week, the Syrian U.N. ambassador, Bashar Ja'afari, claimed extremist groups could use chemical weapons against Syrians and then blame the government.
In Washington, State Department spokeswoman Victoria Nuland said Tuesday that the Obama administration holds the regime in Damascus responsible for securing the chemical weapons. She said that "any effort to abrogate that responsibility, any effort to shift that on to others is just (adding) further to the kind of garbage that we've seen from the regime."
Idris, a 55-year-old German-trained electronics professor, was chosen earlier this month as chief of staff by several hundred commanders of rebel units meeting in Turkey.
With the election of Idris and a 30-member military command center, Syria's opposition hopes to transform largely autonomous groups of fighters into a unified force. The reorganization came after Syria's political opposition won international recognition this month as the sole representative of the Syrian people.
The West has refused to supply Syria's opposition with weapons for fear they could fall into the hands of Islamic militants among the rebels, such as the al-Qaida-inspired Jabhat al-Nusra, which the U.S. designated a terrorist group last week.
Idris said al-Nusra chose not to be part of the rebel command.
He estimated that about a fifth of al-Nusra's fighters are foreigners, but said he believes they will leave Syria once the regime has been toppled. He said the Syrians in the group, which is believed to number several hundred fighters in all, could be brought back to a more mainstream Islam after the war.
"They are not terrorists," he said of al-Nusra.
Speaking in a hotel lobby in the southern Turkish town of Antakya near the Syrian border, Idris said the new military command represents the vast majority of these fighters, and that he has begun taking command inside Syria in recent days. The ex-general said he has set up five regional operations centers, staffing each with about 15 defected army officers.
He said he is frustrated at times with the lack of discipline among the rebels, the vast majority of them civilians without proper military training.
"We need a lot of patience," he said. "If we have a battle, some show up without invitation. They want to take part and shoot."
Idris said that on Tuesday, he spent much the day near the central city of Hama, observing a successful rebel attempt to capture five regime checkpoints.
Syria's conflict began with a popular uprising in March 2011, but quickly turned violent, with protesters taking up arms in response to a brutal regime crackdown. Activists say more than 40,000 Syrians have been killed and aid officials estimate some 3 million people have been displaced by the fighting.
Idris portrayed Assad as a powerless figurehead, saying decisions are made by his inner circle of fellow Alawites, followers of an offshoot of Shiite Islam and a minority in Syria. The opposition is dominated by majority Sunni Muslims.
The ruling elite won't surrender and is willing "to set everything on fire," warned Idris, who served in the military for 35 years, including as dean at the military's technical college in the city of Aleppo, now a major battleground.
U.S. officials have said the Syrian regime launched more than a half-dozen Scud missiles in recent days, the first time it has used such weapons in this conflict. Idris said he was aware of three launches, including two missiles that fell in Syria's eastern desert and a third on the outskirts of a town close to Aleppo.
Idris, citing information from rebel sympathizers within the regime, said Scud missiles are being trained at northwestern Syria, the area close to the Turkish border, and could be fired at any moment.