Syrian rebel groups launch major offensive on Assad positions on Israeli border.

Quneitra sits in a sensitive piece of territory around 70 kilometers (40 miles) southwest of the capital Damascus and has been the scene of frequent fighting between various insurgent groups and the Syrian military backed by allied militia.

Rebel spokesman Issam al-Rayes wrote on Twitter that insurgent groups fighting in the operation had signed a pact, which did not involve al Qaeda's Syria wing Nusra Front.

The groups in the offensive are fighting under the banner of the rebel Free Syrian Army. Nusra has fought in southern Syria but is not thought to be the main insurgent force there, unlike in other parts of the country.

A Reuters photographer watching from the Israeli side of the Golan said there had been heavy shelling since early Wednesday in the Quneitra area, with scores of bombardments.