Prime Minister Tayyip Erdogan said on Thursday Turkey would push for Israel to be tried at an international criminal court if it kept up its assault on Gaza and he accused the Jewish state of "spitting blood".
Meanwhile, a Jewish American group is asking Erdogan to return an award it gave him in 2004, accusing the Turkish leader of "dangerous rhetoric" and "inciting violence against the Jewish people."
Turkey, a member of the US-led NATO military alliance, was once Israel's closest regional ally but has become one of its most vitriolic critics, with Erdogan last weekend accusing it of "surpassing Hitler in barbarism" with its Gaza offensive.
"If Israel continues with this attitude, it will definitely be tried at international courts," Erdogan, who is campaigning for a presidential election on Aug. 10, told a rally of supporters in the southern port city of Mersin.
"We will see this happen and Turkey will struggle for this," he told the cheering crowd.
In an open letter to Erdogan Thursday, Jack Rosen, the president of the Jewish American Congress, said Erdogan had become "arguably the most virulent anti-Israel leader in the world."
Rosen said Erdogan was given the Profile of Courage award in 2004 because he had at the time worked for a peaceful solution to the Mideast conflict and because of his commitment to protecting Jewish citizens in Turkey.
Turkey was once considered by Washington as a credible broker in the Middle East peace process, particularly given its channels of communication with Islamist group Hamas, but that changed as Erdogan adopted an increasingly anti-Israel stance.
"At the moment, Hamas is prepared for everything in order to achieve a ceasefire... (Palestinian President Mahmoud) Abbas is prepared too," Erdogan told CNN news channel in an interview.
"Israel is not even approaching such a thing and is spitting death, spitting blood," he said, in comments translated by the broadcaster from Turkish and aired on Thursday.
He stood by his comments likening Israel's actions to those of Hitler and said the Jewish state was committing genocide.
"It is beyond comprehension that Israel is still defended by the West and the world is silent about it. Therefore we cannot remain silent and we will not be silent," Erdogan said.
His rhetoric plays well with his electoral base of largely conservative Sunni Muslim voters, who he hopes will hand him victory in next month's poll, the first time Turkey's president will have been elected by a popular vote and not by parliament.
There is widespread anger at the civilian casualties caused by Israel's offensive against Gaza, which is aimed at stopping rockets being fired into Israel by militants loyal to Hamas.
On Thursday IDF forces shelled a UN-run school in the northern Gaza strip, the Gaza health ministry said, killing at least 15 people and raising the conflict's death toll to nearly 750.
Israel, which says it has a right to defend itself, has lost at least 32 soldiers in clashes inside Gaza and with Hamas raiders who have slipped under the fortified frontier in tunnels. Palestinian rockets and mortar bombs have killed three civilians in Israel.
Turkish Foreign Minister Ahmet Davutoglu said earlier on Thursday that Turkey was working with international partners to negotiate both a ceasefire and access for humanitarian aid.
"The US, Turkey, Qatar and Egypt have been working for the last five days to ensure an immediate ceasefire," he told Turkish broadcaster AHaber, adding that Ankara was in constant contact with Palestinian leaders.
Israel said last week it was reducing its diplomatic presence in Turkey after protesters pelted its consulate in Istanbul with stones and draped Palestinian flags on the ambassador's residence in Ankara.
The US State Department has called Erdogan's previous remarks likening an Israeli MP to Hitler "offensive and wrong".
The New York-based American Jewish Congress said Erdogan had become the world's "most virulent anti-Israeli leader" and demanded he return an award it gave him a decade ago, partly for his efforts to broker peace between Israel and Palestinians.
Reuters and the Associated Press contributed to this report