The Islamic State militant group has claimed responsibility for a massive terrorist attack in the Indonesian city of Jakarta on Thursday.

Reuters reports only two civilians were killed in the siege on the Muslim nation's capital. Five suicide bombers and gunmen were also killed in the attack.

"A group of soldiers of the caliphate in Indonesia targeted a gathering from the crusader alliance that fights the Islamic State in Jakarta," ISIS said in a statement. They went on to claim 15 people were killed, although this contradicts most media reports.

It took close to three hours before Jakarta's security forces were able to stop the attack, waged near a Starbucks cafe and department store. Authorities traded gunfire with militants who killed themselves in suicidal blasts.

The two victims were reported to be an Indonesian and a Canadian national. Twenty others, including visitors from Germany, Austria, and Algeria, were injured in the attack.

According to CNN, Jakarta police chief Tito Karnavian said the mastermind of the attack was suspected to be a militant named Bahrun Naim. Police spokesman Anton Charliyan said the suspect was not in Indonesia, but had planned and financed the attack from Syria.

Police speculate Naim is currently in the Syrian city of Raqqa, regarded as the de-facto capital of the ISIS terror organization.

Footage of the attack circulated on social media.

Authorities responded quickly after the incident began, arriving in black armored cars, and deploying sniper teams and helicopters around the area.

Experts such as Clarke Jones at the Australian National University said the low death toll was due to "amateurish" weaponry used by the militants. However, the attack still raises alarm, given its similarities to the coordinated strike on Paris last year, which left 130 people dead.

"It's concerning (to have) yet one more day and another attack in another part of the world," Sajjan Gohel of the Asia-Pacific Foundation told CNN. "And one fears that this is potentially becoming the new normal where ISIS affiliates carry out attacks independently from the leadership based in Syria."