A group that has pledged allegiance to ISIS assassinated Saber Siam, a senior Hamas commander, according to reports on Sunday.

The Army of the Islamic State, a Salafist organization in Gaza, placed a bomb on Siam’s car. They also sent out warnings over social media to Gaza residents, telling them to avoid known Hamas buildings, as they are planning more attacks to occur in the coming days.

The group said that they killed Siam because he was “a partner in a declared war against religion and against Muslims, working for the heretical government in Gaza.”

Salafists, such as ISIS or the Army of the Islamic State in Palestine, believe that some Sunni Muslims, such as Hamas, do not go far enough in the war against the West and are therefore heretical. One of ISIS’s primary theological goals is the restoration of “pure Islam,” so their military aims revolve around purging heresies and heretical organizations from their caliphate.

This recent car bombing comes after a mortar attack on a Hamas base in Gaza, perpetrated by a group calling itself the “Supporters of the Islamic State in Jerusalem.”

Haaretz reported that despite this recent increase in violence, Hamas claims that they are not worried about the ISIS-sympathizing radicals, because they are not officially tied to the larger ISIS movement and therefore lack the logistical and financial support necessary to oust Hamas.

However, security forces in Gaza are on high alert. Hamas has been trying to crack down on these local ISIS supporters, setting up more security check-points at night, and imprisoning about 100 accused of sympathizing with Salafist causes, including several imams.

“In the light of Hamas’ new crackdown,” the Supporters of the Islamic State said in a statement, “We renew our loyalty to [ISIS Caliph Abu Bakr] al-Baghdadi and call on him to strengthen his influence and to launch a campaign in Palestine.”

The last time Hamas openly fought local Salafists was in 2009. An imam declared that he was founding a new emirate in Gaza, and in response Hamas stormed his mosque and killed nearly two dozen people.

“The basic dynamics haven’t changed, but the Salafists have gotten bolder,” Jerusalem-based analyst Nathan Thrall told media. “They are now openly attacking and threatening Hamas itself, which is something new.”

If the Salafist threat to Hamas continues to grow, analysts believe that Hamas will be forced into temporarily cooperating with Egypt and Israel—its two primary enemies in the region.