Jakarta attackers linked to IS: police

Lauren Farrow and Heru Rahadi

The two suicide bombers who attacked Indonesia's capital are believed to be linked to Islamic State and could have been inspired by recent attacks overseas, police say.

Indonesian President Joko Widodo has called for calm and unity hours after twin blasts broke out in Jakarta's east on Wednesday night, killing three police officers and leaving 11 people injured.

National Police spokesman Inspector General Setyo Wasisto said the perpetrators were believed to be "related" to Islamic State.

Authorities are trying to identify them through forensic testing and the help of Indonesian anti-terrorism agency Detachment 88.

The attack comes just days after the suicide bombing at Manchester Arena in the UK, which killed 22 people.

"Incidents in other parts of this world could be the trigger or inspiring cells which were hiding on the ground," Ins Gen Wasisto told reporters in Jakarta on Thursday.

"I ask the people, let's come together to fight terrorism. Secondly, don't be afraid."

Police say the fatal events were witnessed by an officer who was helping to smooth the way for a torch festival for the coming Islamic holy month of Ramadan.

"He was eating "pecel lele" (eel with sambal) about 50 metres (from the scene) when he heard the first explosion," Ins Gen Wasisto said.

He ran towards it, smelling a strong odour and seeing thick, white smoke.

He saw four of his colleagues on the ground, including Jakarta Police Second Brigadier Yogi and Second Brigadier Topan.

Five minutes later, about 9.05pm, the second explosion at the Kampung Melayu bus terminal took place.

Police are sifting through evidence from the scene and have a receipt for a pressure cooker bought at a mini-market in Padalarang, West Java.

Mr Widodo called for everyone to "keep calm and maintain unity", adding: "I convey my deepest condolences to the victims' families."

Australian authorities have reminded travellers of the "ongoing high threat of a terrorist attack in Indonesia".

Prime Minister Malcolm Turnbull has spoken with Mr Widodo, offering his support and condolences.

The bombings are the latest in a string of attacks since 2010 that have targeted Indonesian police, who are seen as the enemy due to Detachment 88, which has been the major agency arresting and in some cases killing people involved in pro-Islamic State cells.

Institute for Policy Analysis of Conflict director Sidney Jones said it showed a push to "step up activities" before and during Ramadan.

"It takes on a deeper significance if you undertake a martyrdom action during Ramadan," Dr Jones told AAP.

Twitter hastags #We'reNotAfraid and #PrayforJakarta became popular, with people sending their wishes to the family of victims and calling for unity.

While small-scale suicide attacks are frequent across Indonesia, the bombings are the first in Jakarta for more than a year.

On January 14, 2016 multiple explosions and gunfire broke out in the city's centre, resulting in the deaths of four people and the four attackers.