Three Turkish policemen and three gunmen were killed in an attack on the United States consulate in Istanbul on Wednesday, the city's governor said.

Witnesses told Reuters four attackers drove a car up to the high-walled compound situated to the north of Istanbul city centre and overlooking the Bosphorus waterway. Three jumped out as the car halted and began firing at police who approached.

Governor Muammer Guler said one of the police officers died at the scene in a gunbattle lasting several minutes, at a time of day when many Turks go there to apply for visas. Two had died of their wounds at a nearby hospital.

Two other people were also injured.

Turkey and the United States condemned the 11:00 am (9 a.m. British time) attack for which no one has yet claimed responsibility.

"It is enough to say they are terrorists who carried out a dastardly and cowardly attack," U.S. Ambassador Ross Wilson told a news conference in Ankara.

"Our countries stand together in the fight against international terrorism. This was an attack on a diplomatic establishment here," Wilson said.

Television images showed four bodies lying on the ground around the police post at the consulate's gates, with paramedics carrying out heart massage on one man. The shirt of another was ripped open. Blood was flowing from the head of a third.

"They (the assailants) were four people. Three of them got out of the car and fired at the police. I saw them dead afterwards lying on the ground and many more dead among the police," Enis Yilmaz, who was going to the consulate to get a visa, told Reuters. He said the fourth man drove off.

One of the dead police officers was working at the consulate, and the two others were traffic officers.

"I was greatly saddened by the martyrdom of our three police officers in a terrorist attack," Prime Minister Tayyip Erdogan said in a statement.

The attack coincides with political tensions in Turkey. The ruling party is in a legal fight to avert closure over charges of anti-secular activities and police are probing a shadowy far-right group suspected of plotting a military coup.

Financial markets were not affected by the attack.

WEARING COATS

"We saw four people in a car, they were wearing coats and that seemed pretty weird in this weather. Then we saw they had guns," Muhammet Nur, 15, told Reuters.

"At first we thought they might be civil police but at that moment they drew their guns and a gun battle began," said Nur, who saw the gunbattle from a nearby cafe.

"I could not get the (car) plate number but my friend did."

Istanbul governor Guler said that the three dead gunmen were Turkish citizens, believed to be aged 25-30. Police were searching for a man suspected of driving the car.

State Department spokesman Tom Casey, speaking in Sofia, said no Americans were injured and the United States did not know who was responsible or whether the consulate was a target.

Mutlu Gunes, a 13-year-old eyewitness, said he was on his way to a mosque when he spotted several men preparing guns and placing them inside a Ford Focus car, before driving a short distance to the modern consulate complex.

"The three of them got out of the car. One of them shot a policeman in the chest and I saw one terrorist killing himself after being shot by police. Then I hid under a car," Gunes told reporters.

Turkey has seen armed attacks from a variety of groups over the years, including Maoists, Trotskyists, Kurdish separatists and Islamist militants.

"I curse strongly these kinds of terror attacks. Turkey will struggle to the end with those who organise these (attacks) and the mentality behind them," President Abdullah Gul said.

The U.S. consulate was moved to a high-security location in 2003 as major consulates and embassies stepped up security following the September 11, 2001, attacks in New York.

The most serious attacks in Turkey were in November, 2003, when 62 people were killed by Islamist militants targeting two synagogues, a bank and the British consulate.

Four people were killed and 15 wounded in an explosion in Istanbul in June 2004, before U.S. President George W. Bush visited the city.