Three Turkish police officers were killed today in an attack on the American consulate in Istanbul, in what the US ambassador described as "an obvious act of terrorism".

The officers were standing in front of the compound, just outside Istanbul city centre, when witnesses said they were attacked by gunmen.

Three attackers were seen to jump out of the car as it halted and started firing at police. After several minutes of fighting, three police officers were said to have died while the gunmen were shot dead by Turkish security services who joined the gun-battle.

Turkish television images showed paramedics carrying out a heart massage on one of several pedestrians seen lying on the ground. The shirts of one of the men was ripped open. Another person was seen lying on the ground, with blood coming from a head wound.

"They 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 for a visa application, told Reuters. He said the other attacker sped away in the vehicle.

One of the dead police officers was working at the consulate while the other two were traffic officers.

Ulus Durgut, 24, who was in the process of entering the compound, said the gunbattle lasted 15 minutes. "The terrorists were bearded men and had long hair," Mr Durgut told Reuters.

Mutlu Gunes, a 13-year-old eyewitness, said that 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 to the nearby consulate.

"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," he said.

The secure US consulate building was built after homegrown militants linked to al-Qaeda carried out suicide bombings which targeted two synagogues, the British Consulate and the HSBC bank in Istanbul in 2003, killing 58 people.

Ross Wilson, US ambassador to Turkey, said the attack had been the work of terrorists. Broadcaster CNN Turk said one of the attackers had a Syrian passport. Turkish police, however, refused to confirm the report and said work was still continuing to identify the assailants.

"It’s an obvious act of terrorism," he said. "This was an attack on an American diplomatic establishment. The persons who lost their lives are Turkish citizens and we are very sad about that."

On a day of instability in Turkey, it emerged that Kurdish guerrillas had kidnapped three German tourists on a climbing expedition in eastern Turkey.

The three tourists had established a camp on Mount Ararat in Agri province as part of a 13-member climbing team when they were seized by a group of Kurdistan Workers Party (PKK) militants, the state-run Anatolian news agency reported.

If the PKK is proven to have carried out the kidnapping, it would be a rare tactic for the separatist group whose activities are mainly focused on attacking military targets in southeast Turkey.

Anatolian reported Agri Governor Mehmet Cetin as saying the climbers had arrived in the region three days ago and had established a camp at a height of 3,200 metres (10,500 feet) on the mountain.

Five PKK militants approached the camp and chose three people to kidnap, he said. Their identity was not clear. Agri province, which borders Iran, is to the north of the main PKK conflict region and is a popular destination for mountain climbers.

"The terrorists said they carried out this action because of the German government’s recent moves against PKK associations and sympathisers," Anatolian reported the governor as saying.

Last month Germany banned Kurdish television station Roj TV, which Interior Minister Wolfgang Schaeuble described as being a mouthpiece for the PKK. Germany also extradited two PKK militants to Turkey last year.

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