Three men threw Molotov cocktails at two churches in Makassar, the provincial capital, in the early hours of 10 February.
The front door and sign of the Toraja Mamassa Church of Jordan Congregation were damaged in the attack, but the Tiatira Malengkeri Church was unscathed.
According to Barnabas Aid, two other Makassar churches, the South Sulawesi Indonesian Christian Church and the Toraja Klassis Makassar Church Panakkukang Congregation, were targeted in coordinated attacks at 4am on 14 February. Both buildings suffered fire damage.
It is not known who was behind the bombings, but police have said that they believe the attacks were intended to stir up religious tensions. They are stepping up security in response to the incidents.
South Sulawesi Police Chief Insp. Gen. Mudji Waluyo said: "There are at least two officers . openly guarding each church while another keeps a discreet eye on it. We will also install CCTV at each church."
A spokesperson for Barnabas Aid said, "Makassar has generally been a peaceful place, but there has been a recent spate of suspected terrorist activity in the city. In early January, two members of a Muslim extremist group were killed in a shootout with the police inside a mosque. This followed the targeting by an Islamic group of the governor of South Sulawesi with a crude bomb at a sporting event in November."
Sulawesi and the nearby Moluccas were ravaged in a bloody anti-Christian campaign by Islamic extremists between 1997 and 2001. Hundreds of churches and thousands of homes were destroyed; according to some estimates 30,000 Christians were killed and about half a million driven out in what amounted to ethnic cleansing.
"Conditions for Christians in the area have since eased considerably, but there have been sporadic episodes of violence. The beheading of three girls as they made their way their Christian school in Central Sulawesi in 2005 was among the most egregious," added the Barnabas Aid spokesperson.