President Joe Biden leaves Holy Trinity Catholic Church in the Georgetown section of Washington after attending Mass on Saturday.

Manuel Balce Ceneta/AP

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Manuel Balce Ceneta/AP

President Joe Biden leaves Holy Trinity Catholic Church in the Georgetown section of Washington after attending Mass on Saturday.

Manuel Balce Ceneta/AP

President Biden called Hamas’s attack on Israel “pure unadulterated evil,” as he vowed to “stand with Israel.”

“We’ll make sure Israel has what it needs to take care of itself,” Biden said from the White House.

Hamas militants attacked Israel Saturday, killing more than 900 people, including at least 14 Americans. Israel has responded with a siege and heavy bombing of the Gaza Strip.

“Like every nation in the world, Israel has the right to respond – indeed has a duty to respond – to these vicious attacks,” Biden said.

Biden has expressed full-throated support for Israel in the wake of what he and his officials have described as terrorist attacks. The president spent the long holiday weekend meeting with his national security team and working the phones with allies, including calls with Israeli Prime Minister Benjamin Netanyahu. Biden and Vice President Kamala Harris spoke to Netanyahu ahead of the president’s remarks.

Biden pledged full U.S. support, with aid already on the way to Israel.

On Tuesday, Biden said the U.S. would send military systems to replenish Iron Dome, the Israeli air-defense system.

“When Congress returns, I’m going to ask them to take urgent action to fund the national security requirements of our critical partners,” he said.

The White House is also preparing for what spokesman John Kirby described as the grim possibility that Americans could be among the hostages now being held by Hamas. Biden confirmed Tuesday that Americans were among the hostages being held by Hamas. He did not provide a number.

“As president, I have no higher priority than the safety of Americans being held hostage around the world,” he said.

Earlier, in a statement he ordered his team to work with Israel on hostage recovery efforts.

Separately, U.S. officials tell NPR they are offering Israel air defense and munitions and are pressing to get munitions to the country as quickly as possible. Department of Defense officials offered no timetable on the USS Gerald Ford Strike Group — which includes an aircraft carrier, a guided missile cruiser, and guided missile destroyers — making its way to the Mediterranean.

The president also delivered a warning to other actors in the region.

“To anyone thinking of taking advantage of this situation, I have one word,” he said. “Don’t.”