The fighting is part of a new coordinated offensive in Aleppo by a newly-formed coalition between al-Qaeda's branch in Syria, the Nusra Front, and the ultra-conservative Ahrar al-Sham group, and other rebels. The groups said they seek to "liberate" Aleppo under their coalition called Ansar al-Sharia.
A former industrial and commercial hub, Aleppo has been carved up between government and rebel-held neighborhoods since 2012. With the city devastated by three years of fighting, many of its residents have long fled.
The government struck back with a series of airstrikes and shelling that killed at least 35 militants, according to the Observatory. There was no casualty figure for government troops.
State TV said troops repelled the attacks and aired a live report from the city to show the situation was under control. The formation of Ansar al-Sharia and its push in Aleppo comes amid a string of battlefield losses for President Bashar Assad's overstretched forces in the north and south.
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The TV said the bomb was placed under the pulpit, or minbar, at the Grand Mosque and went off shortly after the midday prayers ended. The report said Sheikh Suleiman Afandi was instantly killed.
It was not immediately clear who was behind the killing. Tal has witnessed reconciliation between the government and rebels but is mostly opposition-controlled.
Bombings targeting mosques have not been uncommon during Syria's civil war. In 2013, Sunni Muslim preacher Sheikh Mohammad Said Ramadan al-Buti an outspoken supporter of President Bashar Assad was killed along with at least 41 others when a suicide bomber struck a Damascus mosque.