Jerusalem, Reuters—Two Palestinians armed with a meat cleaver and a gun killed four people in a Jerusalem synagogue on Tuesday before being shot dead by police, the deadliest such incident in six years in the holy city amid a surge in religious conflict.

Israeli Prime Minister Benjamin Netanyahu pledged to respond with a “heavy hand,” and again accused Western-backed Palestinian President Mahmoud Abbas of inciting violence in Jerusalem.

Abbas issued a statement condemning the attack, which comes after a month of unrest fueled in part by a dispute over Jerusalem’s holiest shrine.

A worshiper who attended the morning service in the Kehillat Bnei Torah synagogue in an ultra-Orthodox neighborhood of West Jerusalem said about 25 people were praying when shooting broke out.

“I looked up and saw someone shooting people at point-blank range. Then someone came in with what looked like a butcher’s knife and he went wild,” the witness, Yosef Posternak, told Israel Radio.

Pictures posted by an Israeli army spokesman on the Internet showed a man in a Jewish prayer shawl lying dead, a bloodied butcher’s cleaver discarded on the floor and several overturned prayer tables.

“We are viewing this as a terrorist attack,” said police spokesman Micky Rosenfeld, who confirmed the four dead and that the two assailants, both from predominantly Arab East Jerusalem, had been shot dead by police.

Israel’s ambulance service said at least eight people were seriously wounded.

In a statement, Abbas said: “The presidency condemns the attack on Jewish worshipers in one of their places of prayer in West Jerusalem and condemns the killing of civilians no matter who is doing it.”

US Secretary of State John Kerry described the attack as an act of “pure terror,” and his spokeswoman said he phoned Netanyahu to offer his condolences.

Palestinian radio reports described the attackers as “martyrs” and the Islamist group Hamas praised the attack. Loudspeakers at mosques in Gaza called out congratulations. However, there was no direct claim of responsibility.

Palestinian media named the attackers as Ghassan and Udai Abu Jamal, cousins from the Jerusalem district of Jabal Mukaber, and said they were members of a small militant group. Clashes broke out in Jabal Mukaber as Israeli security forces moved into the area to make arrests.

“Hamas calls for the continuation of revenge operations and stresses that the Israeli occupation bears responsibility for tension in Jerusalem,” Hamas spokesman Sami Abu Zuhri said.

The synagogue attack came a day after a Palestinian bus driver was found hanged in his vehicle in Jerusalem. Israel said an autopsy showed he committed suicide, findings rejected by his family, which says he was attacked. Hundreds of mourners at the driver’s funeral on Sunday chanted for revenge.

Netanyahu said the attack on the synagogue was “a direct result of incitement” led by Hamas and Abbas, “incitement that the international community has been irresponsibly ignoring.”

“We will respond with a heavy hand to the brutal murder of Jews who came to pray and were killed by lowly murderers,” the Israeli leader said.

Violence in Jerusalem, areas of Israel and the Israeli-occupied Palestinian territories has surged in the past month, fueled in part by a dispute over Jerusalem’s holiest shrine, which has given rise to fears of a religious conflict.

Five Israelis and a foreign visitor have been deliberately run over and killed or stabbed to death by Palestinians. About a dozen Palestinians have also been killed, including those accused of carrying out those attacks.

Residents trace the violence in Jerusalem to July, when a Palestinian teenager was burned to death by Jewish assailants, an alleged revenge attack for the abduction and killing of three Jewish teens by Palestinian militants in the occupied West Bank.

The summer war between Israel and Hamas militants in Gaza and a row over access to a Jerusalem compound that is sacred to Muslims and Jews alike have also been triggers for violence.

The synagogue attack was the worst in the city since 2008, when a Palestinian gunman shot dead eight people in a religious seminary school in the city.

The founder of Israel’s Zaka emergency response service, which attended the scene on Tuesday, described it as like the slaughter of Jews in Europe during the Holocaust.

“The images I saw inside the synagogue reminded me of images from the Holocaust—Jews wrapped in talitot [prayer shawls] and tefilin [phylacteries] dying in pools of their own blood on the floor of the synagogue,” Yehudi Meshi Zahav told Israel Radio.

“These Jewish men were in the middle of their morning prayers when the terrorists entered the building and slaughtered them.”