JAKARTA - Indonesian police on Thursday arrested four suspected militants believed to have been involved in the gun and bomb assault in the centre of the capital, which killed at least six.

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.

Police said four suspected attackers were killed.