ISTANBUL, Turkey — Men armed with pistols and shotguns attacked a police guard post outside the U.S. consulate in Istanbul today, sparking a gunbattle that left three attackers and three officers dead.

Turkish and U.S. officials called the shooting a terrorist attack. The U.S. ambassador to Turkey and Turkey's foreign ministry said security around all American diplomatic missions in Turkey had been increased.

Yavuz Erkut Yuksel, a bystander, told CNN-Turk television the attackers emerged from a white vehicle and surprised the guard.

"One of them approached a policeman while hiding his gun and shot him in the head," Yuksel said.

A fourth policeman and the driver of a towing vehicle were wounded in the attack, he said.

U.S. Ambassador Ross Wilson said the consul general in Istanbul, Sharon Wiener, told him that that consulate staff were "safe and accounted for."

At least two of the attackers were Turkish nationals, Istanbul Gov. Muammer Guler said. Police said they were pursuing a fourth attacker who escaped in a car after the attack outside the high-walled consulate compound in the residential Istinye district around 11 a.m.

Television footage showed four people lying on the ground at the foot of the consulate's wall before officials removed the bodies.

"The Turkish police responded quickly and effectively. We are deeply grateful for the work that they do to protect our official U.S. government establishments here," Wilson said. "It is, of course, inappropriate now to speculate on who may have done this or why. It is an obvious act of terrorism. Our countries will stand together and confront this, as we have in the past."

The secure U.S. consulate building was built after homegrown Islamic militants linked to al-Qaida carried out suicide bombings in 2003 that targeted two synagogues, the British Consulate and a British bank in Istanbul. Those attacks killed 58 people.

"There is no doubt that this is a terrorist attack," said Guler, who described the three slain policemen as "martyred."

The shooting coincided with the visit to Istanbul of top American officials involved in the fight against illegal drugs. Michele Leonhart, acting administrator of the U.S. Drug Enforcement Administration and Scott Burns, deputy director of the White House Office of National Drug Control Policy, were attending an anti-drug conference in another part of Istanbul this morning. It was not clear if they had planned to visit the consulate but visiting U.S. delegations almost always visit diplomatic missions.