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.” U.S. Secretary of State John Kerry said he spoke to Netanyahu and denounced the attack 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.

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.

The Popular Front for the Liberation of Palestine, a small militant group, said the cousins were among its members.

Hamas, the militant Palestinian group that runs the Gaza Strip, praised the attack. In Gaza, dozens took to the streets to celebrate.

Israeli police said six people were also wounded in the attack, including two police officers. Four of the wounded were reported in serious condition.

Much of the recent violence stems from tensions surrounding the Jerusalem holy site referred to by Jews as the Temple Mount because of the Jewish temples that stood there in biblical times. It is the most sacred place in Judaism; Muslims refer to it as the Noble Sanctuary, and it is their third holiest site, after Mecca and Medina in Saudi Arabia.

The site is so holy that Jews have traditionally refrained from going there, but in recent years, a small but growing number of Jewshave begun regularly visiting the site, a move seen as a provocation.