Two men mourn on the ground in front of police officers at the U.S. Consulate in Istanbul on Wednesday. (Ibrahim Usta/Associated Press)

Gunmen opened fire on police guarding the U.S. Consulate in Istanbul on Wednesday, sparking a battle that left six people dead, officials said.

Istanbul Gov. Muammer Guler confirmed that three police officers and three of the attackers were killed in the Turkish city, while another police officer and a tow-truck driver were injured.

Police are now hunting for the fourth attacker who fled in a van, while forensics teams are examining a shotgun left on the grounds at the scene. Police estimated 60 bullets were fired over the course of the gunfight, which lasted several minutes.

"There is no doubt that this is a terrorist attack," Guler said, noting that at least two of the attackers were Turkish nationals.

In Washington, U.S. State Department spokesman Sean McCormack condemned the assault and said the United States was working closely with the Turkish authorities on the investigation into the incident.

Turkey and the United States, McCormack said "will continue to stand firmly together to confront the threat of terrorism as we have done in the past."

Ross Wilson, the U.S. ambassador to Turkey, said no Americans were killed or injured in the attack.

"The police responded effectively and quickly," he said in a statement. "We are deeply grateful for the work that they do to protect our official U.S. government establishments here."

Consulate is highly fortified

The heavily fortified U.S. Consulate was built after the Sept. 11 attacks on the United States. (Murad Sezer/Associated Press)

Wilson said the attackers opened fire on the guard post outside the high-walled consulate's main entrance at about 11 a.m. local time. The building is on a hill in Istinye, a residential neighbourhood in the European side of Istanbul, a city that straddles the European and Asian continents.

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

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

Wilson would not speculate on who carried out the attack or why. It coincided with a visit by two top U.S. government officials, who are attending a conference in Istanbul on illegal drugs.

A police official in Istanbul told the Associated Press that authorities suspected al-Qaeda was behind the attack. The official spoke on condition of anonymity because he was not authorized to brief journalists on the investigation.

The State Department's McCormack said he had no detailed comment to make on the possibility of al-Qaeda involvement.

"At this point, one can't rule that out, but I also can't support at this point, those suspicions," McCormack said. "We're continuing our efforts to work with the Turkish government to determine who is responsible for it."

The fortified U.S. Consulate building was built after the Sept. 11, 2001, attacks on the United States and the 2003 attacks on Istanbul, which saw armed militants carry out suicide bombings that targeted two synagogues, the British Consulate and a British bank. A total of 58 people were killed in the attacks.

U.S. diplomatic buildings and officials have been targeted in countries around the world, with several attacks this year alone.

In March, attackers targeting the U.S. Embassy in Yemen ended up striking a high school for girls next door, killing a security guard and wounding 13 girls. In January, attackers struck a U.S. Embassy vehicle in Lebanon, killing several bystanders and injuring other people.

With files from the Associated Press