ISTANBUL  - Three gunmen and three Turkish policemen were killed Wednesday in an attack outside the well-fortified US consulate in Istanbul that was condemned by US and Turkish officials as a "terrorist" act. The assailants, armed with guns and rifles, targeted a police guard post next to the high-walled mission in the upscale district of Istinye, Istanbul Governor Muammer Guler said. Driving to the front of the consulate in a car, they emerged from the vehicle and opened fire at the post around 11:00am (0800 GMT), witnesses told NTV television, adding that they had also fired shots at the building. The security forces returned fire, killing all three gunmen. A driver escaped in the car after the shoot-out. One policeman died on the spot, while two others succumbed to their injuries in hospital, Guler said, adding that two other people - a policeman and the civilian driver of a police truck - were injured. The attack was "an obvious act of terrorism" aimed at the United States, the US ambassador to Turkey, Ross Wilson, said in Ankara. "The persons who lost their lives are Turkish citizens and we are very sad about that," he said, praising the police response as "effective and quick." All US consulate staff were safe and accounted for, he said. In Washington, the White House also condemned the attack, but would not comment on whether the consulate was the target. US Secretary of State Condoleezza Rice, on a visit to Tblisi, voiced "deep regret" at the loss of life and highlighted the "very rapid and proper response" by Turkish officials. Turkish Interior Minister Besir Atalay said no one had yet claimed responsibility for the incident. Istanbul Governor Guler revealed that the three dead attackers were all Turkish nationals. "We have everything on camera," Guler added. Guns and rifles were seized at the scene after the shootout, which lasted about eight minutes. Television footage showed a bloodied body, covered with newspaper, lying in the street as emergency services rushed the wounded to hospital. A major police operation was underway to catch the fourth attacker who sped away from the scene. "There is a possibility this man may be wounded," Atalay said. The police post was outside the consulate's main public entrance, from where steep steps lead up to the fortified building. Turkish officials also condemned the attack and the investigation was handed to prosecutors specialising in terrorism cases. "I strongly condemn such terrorist acts. Turkey will fight to the end against those who are behind them," President Abdullah Gul said. Since the attack, security measures had already been strengthened at the consulate, said Wilson. Extra measures had also been taken at the US embassy in Ankara and the consulate in the southern city of Adana, which is near a major Turkish air base often used by US aircraft, he added. "We remain a close friend and ally of Turkey. We'll not be deterred in any way by terrorists who are seeking to strike at us or at US-Turkish relations," he said.