Two Palestinians stormed a Jerusalem synagogue on Tuesday, Nov 18, attacking worshippers praying inside with knives, axes and guns, and killing four people before they were killed in a shootout with police, officials said, according to the Associated Press.

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.”

Palestinian President Mahmoud Abbas condemned the attack, the first time he has done so since a recent spike in deadly violence against Israelis began. He also called for an end to Israeli “provocations” surrounding the sacred site.

In a statement, Abbas’ office said he “condemns the killing of the worshippers in a synagogue in west Jerusalem.” The statement, according to the AP, called for an end to the “invasion” of the mosque at the holy site and a halt to “incitement” by Israeli ministers.

Israeli police called the incident a terrorist attack and said the two Palestinian assailants were cousins from east Jerusalem. The Popular Front for the Liberation of Palestine, a militant group, said the cousins were its members. A PFLP statement did not specify whether the group instructed the cousins to carry out the attack. Hamas, the militant Palestinian group that runs the Gaza Strip, praised the attack.

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.

U.S. Secretary of State John Kerry angrily condemned the attack and demanded that the Palestinian leadership take immediate steps to end incitement to violence as Israeli-Palestinian tensions soared.

“This morning in Jerusalem, Palestinians attacked Jews who were praying in a synagogue,” Kerry said shortly after Israeli authorities reported that two Palestinians had stormed the synagogue, attacking worshippers with knives, axes and guns, and killed four people before being killed in a shootout with police.