The two rival Palestinian factions have failed to heal their political disputes since Hamas fighters expelled Fatah from Gaza in a short war in 2007
SOHR director Rami Abdel Rahman said he expected the toll in the strike carried out late Tuesday to increase as there were several people with serious injuries.
According to the observatory, over the past three months the Syrian authorities loyal to President Bashar al-Assad have launched 5,012 attacks on civilian targets and at least 1,057 people, including 239 children, have died in the airstrikes.
More than 200,000 people have been killed since the civil war began in Syria in March 2011, according to the UN.