EU chief Ursula von der Leyen on Wednesday said there is "no excuse for hitting a hospital full of civilians" in Gaza, but did not apportion blame for the blast.
The European Commission president told EU lawmakers the "facts need to be established" on the overnight strike on the Gaza hospital which killed at least 200.
Israel and Palestinians accuse each other for the blast, which has triggered street protests in the Middle East against Israel.
Von der Leyen, speaking before the European Parliament in Strasbourg, said the overnight explosion turned the hospital into "a hell of fire".
"All those responsible must be held accountable," she said.
Von der Leyen, who visited Israel last Friday in a show of solidarity with Prime Minister Benjamin Netanyahu's government, has been criticised by some European Union countries for perceived bias to the Israeli side at the expense of Palestinian civilians.
That prompted a summit of EU leaders by videoconference late Tuesday to emphasise the bloc's twin message: that Israel has the right to defend itself following Hamas's bloody October 7 assault -- but only in line with its commitments under international humanitarian law.
Hamas on October 7 sent fighters through the Gaza Strip's heavily militarised border, killing more than 1,400 people. They also took nearly 200 hostages.
Israel has been relentlessly bombing Gaza in response, killing over 3,000 people, according to the health ministry in the Hamas-controlled territory.
In her parliamentary address, von der Leyen said "we must redouble our efforts to protect citizens from the fury of war".
But she said Hamas was the underlying reason for the ordeal Palestinians are now going through.
"Hamas are terrorists. And the Palestinian people are also suffering from that terror," she said.
She added that the EU needs to keep supporting the Palestinians, "and there is no contradiction in standing in solidarity with Israel".
"Europe stands with Israel in this dark moment," von der Leyen said, adding that "Israel should act as a democracy, in line with international humanitarian law".
- Against 'rules of war' -
The EU's top foreign policy official, Josep Borrell, took a sterner tone on the scale of the Israeli reaction, which is affecting all 2.3 million Palestinians in Hamas-controlled Gaza, which Israel has put under siege, cut off from water, food, electricity and humanitarian aid.
"Yes, we condemn these terrible terrorist attacks, but I think we also have to condemn the fact of civilian victims," Borrell told the parliament.
"Cutting water supplies and food off from civilian populations isn't in line with the rules of law," he said.
"We cannot make the people of Gaza responsible for the terrible actions of Hamas."
The speeches to the parliament, and Tuesday's videoconference EU summit, highlighted divergences within the EU over developments of the Israel-Hamas conflict.
While there has been widespread condemnation of the slaughter by Hamas of Israelis, disagreements over calls to rein in retaliatory strikes on Gaza continue to boil.
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