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WATCH: White House expresses ‘serious concern’ over Israel unrest

The White House said Monday it is “closely monitoring” the violence in Israel and said President Joe Biden has “serious concerns about the situation.”

Watch Psaki’s remarks in the video player above.

Speaking at the White House briefing, press secretary Jen Psaki said the president is “kept abreast and is watching” developments carefully.

Psaki spoke amid one of the bloodiest days of battle between Israel and Hamas in several years.

Hamas militants fired dozens of rockets into Israel, after hundreds of Palestinians were hurt in clashes with Israeli police at a flashpoint religious site in Jerusalem.

The rocket fire drew heavy Israeli retaliation in the Gaza Strip. Health officials said at least 20 people, including nine children, were killed in the fighting.

Psaki said national security adviser Jake Sullivan spoke over the weekend with his Israeli counterpart, Meir Ben-Shabbat, and they “agreed that the launching of rocket attacks and incendiary balloons from Gaza towards Israel is unacceptable and must be condemned.”

A White House statement said that Sullivan called on Israel to “pursue appropriate measures to ensure calm.”

Psaki also talked about Biden’s participation in the “Bucharest Nine,” summit Monday.  The group represents the easternmost members of the Western military alliance.

Most of the nine nations share a concern about Russia’s attempts to reassert its influence over their region, their anxieties growing after Russia annexed Ukraine’s Crimean Peninsula in 2014.

Psaki said Biden “underscored his commitment to rebuilding alliances” and stressed his desire to cooperate with the nine allies on a range of issues, including global health security, climate change and energy security.

She said “he also expressed his support for enhancing NATO’s deterrence and defense posture, as well as the importance of allies increasing their resilience against harmful economic and political actions by our strategic competitors.”

Romania’s president told Biden more NATO troops are needed on the alliance’s eastern flank given Russia actions that are perceived as threatening by many in the region.