(NewsNation) — President Joe Biden strongly condemned Hamas’ attack on Israel, which has left hundreds of civilians dead, and pledged U.S. support to Israel in the fight.

Fourteen U.S. citizens have been killed in the violence and 20 Americans are missing. Biden said his administration has offered support in hostage recovery efforts and confirmed, for the first time, that American citizens are among the hostages.

Biden referred to the attack as “pure, unadulterated evil” and compared the group’s actions to ISIS. He also rejected the idea Hamas stands for the Palestinian people seeking statehood, noting that the group has been known to use civilians as human shields.

“Let me be crystal clear: We stand with Israel,” Biden said.

Biden said his administration has been in constant communication with its Israeli counterparts and pledged to ensure that Israel does not run out of assets needed to defend the country and to offer additional assistance to replenish the Iron Dome missile defense system.

The president also called for Congress to take urgent action when it is back in business. The president is limited in what military aid he can offer without approval from Congress; however, the House of Representatives will be unable to bring bills to the floor or hold votes until a new Speaker of the House is elected.

One action the president has taken is to move the USS Gerald R. Ford carrier strike group to the eastern Mediterranean to strengthen deterrence. The president also had a strong warning for those who might take advantage of the unrest.

“To any countries that would take advantage of the situation, I have one word for you,” Biden said. “Don’t.”

While Biden did not specifically mention any countries, Iran and Israel are long-standing adversaries and Iran has been known to back Hamas as well as other terrorist groups in nearby countries including Hezbollah in Lebanon and militias in Syria.

There have been conflicting reports over Iran’s involvement in the attack and the U.S. has not officially confirmed or denied Iranian involvement.

The president did not hold any public events Monday, drawing criticism from some Republicans. The White House responded by saying Biden was meeting with members of his administration and speaking with close allies.

In a statement issued Monday, Biden said U.S. support for Israel is “unwavering,” and described the two countries as “inseparable partners.”

Tuesday marks the fourth day of a war declared after Hamas militants broke through Israeli blockades and attacked communities near the Gaza Strip. Israeli Prime Minister Benjamin Netanyahu declared war on Hamas, vowing to lay total siege to Gaza.

Already, more than a thousand people have died on both sides and thousands more are injured as international organizations call for humanitarian channels to be opened to provide needed aid for civilians caught in the violence.