WASHINGTON — President Joe Biden said Wednesday he warned Iran to “be careful” in the wake of attacks by militant organization Hamas on Israel over the weekend.

After listing aid the U.S. is sending to Israel, and noting that a U.S. carrier fleet was moved to the area, Biden said he “made it clear to the Iranians: Be careful.”

U.S. intelligence showed Iranian leaders were surprised by Hamas’ attack on Israel, sources told NBC News, but the Biden administration has yet to make a definitive conclusion. White House National Security Advisor Jake Sullivan said Tuesday that Iran was “complicit” in the attack because it has backed Hamas for decades.

The president made his remarks Wednesday at a roundtable of Jewish leaders at the White House. The event on antisemitism was scheduled before the attacks took place but took on a different tone in its wake.

Biden said he spoke with Israeli Prime Minister Benjamin Netanyahu again Wednesday morning. During the call, he stressed that Israel must “operate by the rules of war.”

“I’ve known Bibi for over 40 years. We have a very frank relationship. I know him well,” Biden said. “And the one thing I did say is that it is really important that Israel, in all the anger and frustration that exists, operate by the rules of war. And there are rules.”

Biden said he believes the Israeli government is “doing everything in its power to pull the country together” and that the U.S. is doing everything in its power to ensure Israel succeeds.

The president called the attacks “a campaign of pure cruelty.”

“I would argue it’s the deadliest day for Jews since the Holocaust,” Biden said.