EU foreign policy chief Josep Borrell said Saturday that a plan by Israel to evacuate more than one million people out of northern Gaza in a single day was "utterly impossible to implement".
Israel warned residents in the area to evacuate before an expected ground offensive against the Gaza Strip.
Israel has besieged and indiscriminately bombed the Palestinian enclave after a surprise Hamas attack which killed hundreds of Israelis last Saturday.
"I am saying that, representing the official position of the European Union... (the evacuation plan) is utterly, utterly impossible to implement," Borrell told a press conference in Beijing on the final day of a three-day diplomatic visit to China.
"To imagine that you could move one million people in 24 hours in a situation like Gaza can only be a humanitarian crisis," he added.
EU chief Ursula von der Leyen had expressed support to Israeli Prime Minister Benjamin Netanyahu on Friday during a visit to the country.
"Europe stands with Israel," said von der Leyen, adding that the country had "a duty to defend its citizens".
But Borrell's comments in Beijing showed the limits of that affirmation, saying that Israel was also obligated to follow international humanitarian law in the process of defending itself.
"The position is clear," said Borrell. "We certainly defend the right of Israel to defend itself."
"But, as any right, it has a limit. And this limit is international law."
Borrell's words come one week after Hamas gunmen burst through the heavily militarised border around the Gaza Strip and killed more than 1,300 people, mainly civilians.
Israel has responded with an intense bombing campaign against Palestinian targets in Gaza, killing at least 2,200 people -- most of them civilians and including more than 600 children -- according to the Gaza health ministry.
This has triggered calls from some in the international community for Israel to show restraint and to allow humanitarian corridors to be opened for Palestinian civilians.
Borrell said Saturday that the only feasible long-term strategy for resolving what he called a "spiral of violence" was to pursue the creation of an independent and internationally recognised Palestinian state.
China's reaction to the conflict, which did not include a direct condemnation of the Hamas attacks, had been criticised by Western leaders as too weak.
Following a series of high-level talks with Chinese leaders, Borrell said that the EU and China agree on the two-state solution.
"We certainly agree that the only long-term solution to these crises that come one after the other... is to work on the solution of two states," said Borrell.
"We agreed the international community should do the utmost to prevent the further degradation of the situation that could spill over the region."