Two Palestinians stormed a Jerusalem synagogue on Tuesday, attacking worshipers praying inside with knives, axes and guns, and killing four people before they were killed in a shootout with police, officials said.

The attack, the deadliest in Jerusalem in years, is bound to ratchet up fears of sustained violence in the city, already on edge amid soaring tensions over a contested holy site.

Prime Minister Benjamin Netanyahu vowed that Israel will “respond harshly” to the attack, describing it as a “cruel murder of Jews who came to pray and were killed by despicable murderers.” U.S. Secretary of State John Kerry said he spoke to Netanyahu after the assault and denounced it as an “act of pure terror and senseless brutality and violence.”

Israeli police called it a terrorist attack and said the two Palestinian assailants were cousins from east Jerusalem. It was not immediately known if they were affiliated with any group. Hamas, the militant Palestinian group that runs the Gaza Strip, praised the attack but stopped short of claiming responsibility.

Police spokesman Micky Rosenfeld said six people were also wounded in the attack, including two police officers. Four of the wounded were reported in serious condition. He said police were searching the area for other suspects.

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