The Observatory says more than 36,000 people have died since the uprising against al-Assad’s rule broke out in March last year, first as a protest movement inspired by the Arab Spring and then as an armed rebellion.
The exiled opposition Syrian National Council (SNC), which backs the rebels, met in Qatar under intense US pressure to reform.
Its leader, Abdel Basset Sayda, insisted the bloc must remain the “cornerstone” of the Syrian opposition, rejecting US accusations it is unrepresentative.
Opposition figures were to discuss an initiative by leading dissident Riad Seif, reportedly backed by the US, to broaden the opposition beyond the SNC and form a government-in-exile.
International efforts to halt the violence have been frustrated by disagreements, with Russia and China blocking attempts by Western and Arab governments to put more pressure on al-Assad.
Moscow has accused Western governments of conniving in the delivery of weapons to the rebels by Gulf Arab states.
Russian Foreign Minister Sergei Lavrov said on Tuesday that the rebels had acquired 50 Stinger shoulder-fired missiles.
Speaking after talks in Jordan, Lavrov said the missiles were in no way “intended for defense” and could be used to target regime aircraft or civilian planes.