In the most serious attack in Jerusalem in years, two men armed with axes, knives and firearms stormed a synagogue Tuesday, killing at least four worshipers and injuring others before a police officer shot and killed the assailants, authorities said. Israeli police told the Associated Press that three Americans and one British citizen, all of whom also held Israeli citizenship, were killed by the attackers. Palestinian media named the attackers as Udai Abu Jamal and Ghassan Abu Jamal, cousins from the Palestinian village of Jabel Mukabar in south Jerusalem. Fresh clashes broke out Tuesday as police arrived to arrest several of their relatives. According to eyewitnesses and initial Israeli news reports, the attackers burst into the four-story synagogue in the devoutly religious western Jerusalem neighborhood of Har Nof, attacking a crowd attending early morning prayers. Israeli media cited witnesses who said worshipers were felled in mid-prayer, wrapped in their prayer shawls and lying on the floor in pools of blood. A prominent rabbi was among the victims. Prime Minister Benjamin Netanyahu was convening a special security meeting and said Israel would "respond harshly" to what he called "the cruel killing of Jews ... by despicable murderers." In a phone call to Netanyahu, U.S. Secretary of State John F. Kerry expressed condolences for the attack and condemned it as senseless brutality. Kerry was in neighboring Jordan for talks aimed at calming tensions in the region, and met separately last week with Netanyahu and Palestinian Authority President Mahmoud Abbas. A demand from Kerry for Palestinian condemnation of the attack prompted a statement from Abbas, who denounced "killing civilians from any side," including the Jewish worshipers. At the same time, Abbas demanded a halt to what he called Israeli "intrusions" into a contested holy site in Jerusalem's old city. In the Gaza Strip, the attack was welcomed with celebratory gunfire and a statement from Hamas spokesman Sami Abu Zuhri, who called it a response to the "continuing crimes of the occupation." Another official, Hussam Badran, called it a "heroic act." Though recent Israeli-Palestinian tensions have been punctuated by lone-wolf-type attacks, such as stabbings, Tuesday's assault represented a marked escalation, chillingly reminiscent of large-scale strikes in Israeli cities during two previous Palestinian uprisings during the period from the late 1980s to the early 2000s. In Har Nof, those living near the synagogue were told to remain indoors while authorities ruled out the presence of another assailant. Eventually the all-clear was given as the presence of a third attacker was ruled out. Helicopters circled overhead and windows glowed with reflections of the flashing lights of emergency vehicles. Residents in ultra-Orthodox garb filled the streets, wandering among the emergency personnel. The attack was the latest in a wave of violence sweeping Jerusalem and other locations against a backdrop of friction over the holy site revered by Jews as the Temple Mount and known to Muslims as the Noble Sanctuary. Other issues, such as stepped-up plans for Jewish buildings in traditionally Arab east Jerusalem, plans to enlarge West Bank settlements and the summer's war in the Gaza Strip, have also generated fury and resentment among Palestinians. The fact that the attack took place at a Jewish house of worship was certain to further inflame religious tensions. At least six people have died in attacks over the last month, including four people killed by vehicle attacks at tramway stations in Jerusalem, a soldier who was stabbed to death in Tel Aviv and a Jewish woman settler who was killed in the West Bank. This month has also seen fiery attacks on a northern Israeli synagogue and a West Bank mosque that caused property damage but no loss of life or injuries. "We're at war," Israeli lawmaker Aryeh Deri, a resident of the typically quiet ultra-orthodox Jewish neighborhood where the most recent attack took place, told Israel radio. If every synagogue in the country needs protection, we have returned to bad times. Staff writer King reported from Gaza City.