GAZA CITY (Ma'an) -- At least 21 Palestinians were killed in new Israeli strikes on Gaza late on Saturday, medics in the coastal enclave said.

In Gaza City, 18 Palestinians were killed in an Israeli airstrike on the Tuffah neighborhood that hit a house and mosque, emergency services spokesman Ashraf al-Qidra said, making it the single most deadly attack since Israel's assault began.

Another 35 were wounded in the same strike, and three more people were killed in southern Gaza's Rafah that also injured five people, Qidra added.

Among the wounded was police chief Tayseer al-Batsh, officials said.

The strikes came after Hamas launched a barrage of long-range rockets at Israeli cities including Tel Aviv, after issuing a warning that it would strike.

The new deaths brought the toll in the fifth day of the conflict to over 151, with more than 1,000 people wounded.

The 15-member UN Security Council urged a return to "calm, and restitution of the November 2012 ceasefire", referring to Gaza's last deadly full-scale conflict.

On Friday, the United Nations' Office for the Coordination of Humanitarian Affairs said a majority of those killed in Gaza so far -- 77 percent -- were civilians.

Israel's aerial campaign -- the largest and deadliest since 2012 -- saw strikes start early on Saturday, including one that hit a center for the handicapped, and another that killed two nephews of Gaza's former Hamas premier, Ismail Haniyeh.

AFP contributed to this report