The deaths of the World Central Kitchen workers threatened to set back efforts by the US and other countries to open a maritime corridor for aid from Cyprus to help ease the desperate conditions in northern Gaza.

President Joe Biden issued an unusually blunt criticism of Israel by its closest ally, suggesting that the incident demonstrated that Israel was not doing enough to protect civilians.

"Israel has not done enough to protect aid workers trying to deliver desperately needed help to civilians," he said, adding he was "outraged and heartbroken" by their killings.

"Incidents like yesterday's simply should not happen," he added. "The United States has repeatedly urged Israel to deconflict their military operations against Hamas with humanitarian operations, in order to avoid civilian casualties."

Ships still laden with some 240 tons of aid from the charitable group turned back from Gaza just a day after arriving, according to Cyprus. Other humanitarian aid organizations also suspended operations in Gaza, saying it was too dangerous to offer help. Israel has allowed only a trickle of food and supplies into Gaza's devastated north, where experts say famine is imminent.

The dead from Monday night's strikes included three British citizens, Polish and Australia nationals, a Canadian-American dual national and a Palestinian. Those countries have been key backers of Israel's nearly 6-month-old offensive in Gaza, and several of them denounced the killings.

Israel already faces growing isolation as international criticism of the Gaza assault has mounted. On the same day as the deadly airstrikes, Israel stirred more fears by apparently striking Iran's consulate in Damascus and killing two Iranian generals. The government also moved to shut down a foreign media outlet - Qatari-owned Al Jazeera television.

The hit on the charity's convoy also highlighted what critics have called Israel's indiscriminate bombing and lack of regard for civilian casualties in Gaza.