"The number of martyrs in the Al-Shati massacre has risen to 20, and there are still a number of wounded in serious condition," Mahmud Basal, spokesman for the civil defense agency in Gaza told AFP.
There was no immediate comment from the Israeli military.
Palestinians were seen sifting through the flattened remnants of the makeshift mosque after the deadly strike west of Gaza City.
Heavy fighting has raged for weeks in and around Gaza City.
In the Gaza Strip's south on Saturday, Israel said it targeted Hamas's military chief Mohammed Deif.
Hamas has denied the claim as nonsense, saying the attack killed civilians.
The health ministry said at least 90 people were killed in Al-Mawasi, half of them women and children, with another 300 wounded, many in "critical" condition.
Israel has designated Al-Mawasi a "safe zone", ordering Gazans in May to head there from other areas.
Most of Gaza's 2.4 million population has been displaced by the war, now in its 10th month.
The Israeli regime, flouting a UN Security Council resolution demanding an immediate ceasefire, has faced international condemnation amid its continued brutal offensive on Gaza since an Oct. 7, 2023 retaliatory attack by the Palestinian resistance movement Hamas, dubbed Operation Al-Aqsa Flood.
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More than 38,000 Palestinians have since been killed, mostly women and children, and around 87,300 others injured as a result of Israel’s genocidal attacks, according to local health authorities.
Over eight months into the Israeli war, vast tracts of Gaza lie in ruins amid a crippling blockade of food, clean water and medicine.
Israel stands accused of genocide at the International Court of Justice, whose latest ruling ordered it to immediately halt its military operation in the city of Rafah, where over a million Palestinians had sought refuge from the war before it was invaded on May 6.
Source: AFP