JERUSALEM

Two Palestinian cousins stormed a Jerusalem synagogue today, attacking worshippers with meat cleavers and a gun during morning prayers and killing four people before they were shot dead by police.

The attack, the deadliest in Jerusalem in six years, ratcheted up fears of sustained violence in the city, which is already on edge amid soaring tensions over its most contested holy site.

Prime Minister Benj’amin Netanyahu vowed to “respond harshly,” describing the attack 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. 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 called for an end to the “invasion” of the mosque at the holy site and a halt to “incitement” by Israeli government ministers.

The attack was the deadliest in Jerusalem since a Palestinian assailant killed eight students at a Jewish seminar on March 6, 2008.

Israeli police called the incident a terrorist attack and said the two Palestinian assailants were cousins from east Jerusalem, the section of the city captured by Israel in 1967 and claimed by the Palestinians as their capital.

Weeks of unrest have been focused around a contested holy site in the Old City, located in east Jerusalem.

The Popular Front for the Liberation of Palestine, a small militant group, said the cousins were among its members. It 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. In Gaza, dozens took to the streets to celebrate. Some people held trays full of sweets and distributed them to drivers and passersby. In the southern town of Rafah, women and schoolchildren waved green Hamas flags and a loudspeaker praised the attack.

Israeli 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.

The attack occurred in Jerusalem’s Har Nof neighborhood, an ultra-Orthodox community on the western edge of the city far from other recent scenes of violence. The area was surrounded by police and rescue workers following the attack.

Associated Press footage showed wounded worshippers being assisted by paramedics, and a bloodied meat cleaver lay nearby. Initially, police had described the weapons used as knives and axes.

Yosef Posternak, who was at the synagogue at the time of the attack, told Israel Radio that about 25 worshippers were inside when the attackers entered.

“I saw people lying on the floor, blood everywhere. People were trying to fight with (the attackers) but they didn’t have much of a chance,” he said.

Footage released by the Israeli government showed blood-soaked prayer books and prayer shawls on the floor of the synagogue. A pair of glasses lay under a table, from which blood was dripping. A set of phylacteries hung off the table. A photo in Israeli media showed a body on the floor, covered with a prayer shawl.

Police spokeswoman Luba Samri said the attackers were Palestinians from east Jerusalem, which has been the scene of relentless clashes between Israeli police and Palestinian protesters in recent months. She identified the assailants as Ghassan and Oday Abu Jamal from the Jabal Mukaber neighborhood.

Soon after the attack, clashes broke out outside the Abu Jamals’ home, where dozens of police had converged. Residents hurled stones at police who responded using riot dispersal weapons.

Residents in the neighborhood, speaking on condition of anonymity for fears for their own safety, said 14 members of the Abu Jamal family were arrested.

Mohammed Zahaikeh, a social activist in Jabal Mukaber, said a relative of the cousins had been released in a 2011 prisoner swap and rearrested recently by Israeli police. He did not say why.

Israel has been on edge with a spate of attacks by Palestinians against Israelis, killing at least six people in Jerusalem, the West Bank and Tel Aviv in recent weeks before today’s casualties.

The violence has created a special security challenge for Israel, since most of the attackers come from east Jerusalem. Israel annexed the area after capturing it in 1967, and its more than 200,000 Arab residents hold residency rights that, in contrast to Palestinians in the neighboring West Bank, allow them to move freely throughout Israel.

Rosenfeld said an added challenge is that the attackers all appear to have acted on their own, making it difficult to stop them ahead of time. He said police were stepping up patrols in certain Arab neighborhoods and increasing intelligence-gathering activities.

Jerusalem residents had already been fearful following a series of attacks in recentw weeks, but today’s synagogue assault appeared to cross a dangerous line, both in terms of the death toll and the target.

Israel’s police chief said today’s attack was likely not organized by militant groups, similar to other recent incidents, making it more difficult for security forces to prevent the violence.

“These are individuals who decide to do horrible acts. It’s very hard to know ahead of time about every such incident,” Yohanan Danino told reporters at the scene.

Tensions appeared to have been somewhat defused last week following a meeting between Netanyahu, Kerry and Jordan’s King Abdullah II in Amman. The meeting was an attempt to restore calm after months of violent confrontations surrounding a sacred shrine holy to both Jews and Muslims.

Israel and the Palestinians said then they would take steps to reduce tensions that might lead to an escalation.