Jerusalem (AFP) - Israeli Prime Minister Benjamin Netanyahu on Friday vowed harsh retribution against Hamas, the Islamist rulers of Gaza, after a mortar round fired from the Palestinian territory killed an Israeli child.

"Hamas will pay a heavy price for this attack," Netanyahu's spokesman Ofir Gendelman cited the premier as saying on his Twitter account, adding that the Israeli army and Shin Bet internal security service would "intensify ops against Hamas until the goal of #ProtectiveEdge is achieved".

The boy was the first child to be killed on the Israeli side since the July 8 start of the war with Hamas, which Netanyahu has said was aimed at ending rocket fire at Israel and delivering a significant blow to Hamas.

Three other civilians, one a Thai labourer, have been killed in Israel during the conflict, as well as 64 soldiers in and around the Gaza Strip.

"Netanyahu sends his condolences to the family of the 4-year-old boy that was killed this afternoon by a mortar round fired by Hamas," Gendelman wrote.