At least 18 Syrian rebels and a child were killed in violence in the central province of Hama Tuesday, a monitoring group said.

In the north, a prominent Aleppo-based activist went missing, and the Syrian Observatory for Human Rights said it feared the jihadist Islamic State of Iraq and the Levant (ISIS) was behind his disappearance.

Near Morek in Hama province, "the number of opposition fighters killed (early Tuesday) has risen to 18," said the Britain-based Observatory.

They were killed in clashes and army shelling on their positions and ammunition stockpiles, the group added.

Meanwhile, in the Sahel al-Ghab area of Hama, a 10-year-old child was killed in army shelling, said the Observatory.

The deaths come amid a major rebel escalation in eastern Hama, which is strategically located in the heart of Syria and links several flashpoint areas to each other.

Activists say several rebel groups have joined forces in recent weeks to fight two simultaneous battles in the area -- one called "We are coming, Homs", the other "The single body."

Hama is adjacent to Homs, the centre of whose provincial capital remains under rebel control, weeks after the army reclaimed control of the strategic Khaldiyeh district.

In northern Syria, news broke of the disappearance of an anti-Assad activist known for his chants at weekly protests in the city of Aleppo.

"It is likely he is being held by ISIS," Observatory director Rami Abdel Rahman told AFP.

Amateur video distributed by the Observatory showed friends of Abu Maryam protesting at the gates of ISIS' headquarters.

ISIS is believed to be holding hundreds of activists in areas where they operate.

Among the missing is Father Paolo Dall'Oglio, a charismatic Italian Jesuit priest who went to meet with ISIS' commanders earlier this month to try to negotiate the release of activists, and has yet to return.