JAKARTA: Militants launched a gun and bomb assault in the centre of the Indonesian capital on Thursday, killing at least six people, in an attack that followed a threat by Islamic State fighters to put the country in their "spotlight", police said.

Media said six bombs went off and a Reuters witness saw police exchanging fire with gunmen. Several hours after the attacks began, the witness heard more gunfire and at least one more explosion.

One blast was in a Starbucks cafe and security forces were later seen entering the building.

"The Starbucks cafe windows are blown out. I see three dead people on the road. There has been a lull in the shooting but someone is on the roof of the building and police are aiming their guns at him," said a Reuters photographer.

Police suspected a suicide bomber was responsible for at least one of the blasts and up to 14 militant gunmen were involved in the attack, Metro TV reported.

Police snipers were deployed among hundreds of other security officers, some in armoured vehicles. A bomb disposal unit was seen entering the building where the Starbucks is located, which also houses a cinema.

An office worker in a building above the Starbucks cafe, who declined to be identified, said he and fellow workers had been ordered to stay put after the first blast.

"That's when I heard the second explosion. It was loud and powerful," he said.

Indonesia has been on edge for weeks over the threat posed by Islamist militants and counter-terrorism police have launched a crackdown on people with suspected links to Islamic State.

"We have previously received a threat from Islamic State that Indonesia will be the spotlight," police spokesman Anton Charliyan told reporters. But he said police did not know who was responsible.

He said three policemen and three civilians had been killed.

Media reported that a Dutch person and another foreigner were among the casualties but it was not clear if they were dead or wounded.

Police said four suspected attackers were killed.

The last major militant attacks in Jakarta were in July 2009, with bombs at the JW Marriott and Ritz Carlton hotels.

President Joko Widodo was outside Jakarta when the attack unfolded but was cutting shot his trip to return to the sprawling capital of more than 10 million people by helicopter.

He urged the public not to be fearful and or speculate on who was behind the attack.

"We must not be afraid, we must not be defeated by an act of terror like this," he said in televised comments.

The national intelligence agency chief said there was no indication that Islamic State militants had carried out the attack.