Armed men opened fire Wednesday on Turkish police guarding the front entrance to the US consulate in Istanbul, killing three policemen in what Turkish and US officials denounced as a terrorist attack. Three of the assailants were also reported killed. US Ambassador to Turkey Ross Wilson and Turkey's foreign ministry said security around all US diplomatic missions in Turkey had been immediately increased after the attack, which had begun about 11 a.m. on the high-walled compound in the Istinye district. All US consulate staff were safe and accounted for, Wilson said. The level of alert at the Israeli consulate in Istanbul and the embassy in Ankara was raised Wednesday afternoon following the attack, however additional security guards were not dispatched to Turkey. There was no immediate claim of responsibility but Turkish police said they suspected al-Qaida was behind the attack. Interior Minister Besir Atalay said police would not reveal the identities of the slain attackers or their possible affiliations due to the ongoing investigation. Four armed men emerged from a gray car about 11 a.m. and shot a traffic policeman, then ran toward a guard post at the entrance to the consulate and shot at policemen there, according to a security video. Police fired back at the men, killing three of them. At least one of the assailants escaped the chaos in the car. US security personnel then ducked inside the compound, since they are not authorized to engage in armed combat on Turkish soil. Police are pursuing an unknown number of attackers who escaped. NTV television, citing police sources, said police feared the car might be loaded with explosives, but police would not confirm that report. Istanbul prosecutor Aykut Cengiz Engin said the attackers were armed with pistols and shotguns. Forensic teams later examined a shotgun on the ground. "There is no doubt that this was a terrorist attack," Istanbul Gov. Muammer Guler told reporters at the scene, calling the three slain policemen martyrs. Two other people, a policeman and a truck driver, had been injured, he said. Yavuz Erkut Yuksel, a bystander, told CNN-Turk television that the gunmen had surprised the guard. "One of them approached a policeman while hiding his gun and shot him in the head," Yuksel said. Guler said two of the attackers were Turkish, denying speculation that they had Syrian passports. The US ambassador refused to speculate on the identity of the attackers but stressed that the United States and Turkey would keep working together against terrorism. "It is, of course, inappropriate now to speculate on who may have done this or why. It is an obvious act of terrorism," Wilson said. "Our countries will stand together and confront this, as we have in the past." He also praised Turkish police for responding quickly to the attack. The secure US consulate building was built after home-grown Islamic militants linked to al-Qaida killed 58 people in 2003 with suicide bombings of synagogues, the British Consulate and a British bank in Istanbul. Dogan identified one of Wednesday's attackers as 26-year-old Erkan Kirgiz from the southeastern city of Bitlis. Police would not confirm the report.