At least six people were killed and 21 others injured in an explosion during Friday Prayer at a mosque in the Syrian city of Homs, Syria’s interior ministry said.

The blast struck the Imam Ali Ibn Abi Talib Mosque in the Wadi al-Dahab neighborhood, the ministry said in a statement on social media.

Preliminary investigations indicated that the explosion was caused by a device that had been planted inside the mosque, according to a report by Syria’s state news agency, Sana.

The attack happened on a day when many worshipers go to mosque for Friday Prayer. No group has yet claimed responsibility for the blast, though Syria’s interior ministry called it “a terrorist explosion.”

Photos and video published by Sana showed debris scattered across the mosque’s carpets, black soot on the walls and red stains on the ground that appeared to be blood.

The ministry said in its statement that internal security forces had imposed a cordon on the mosque and had begun “investigating and collecting evidence to prosecute the perpetrators of this criminal act.”

The explosion underscored the mounting security challenges facing the country a year after the ousting of the dictator Bashar al-Assad.

Syria’s foreign ministry said in a statement that the explosion was a “cowardly criminal act” that represented “desperate attempts to undermine security and stability and spread chaos among the Syrian people.”

Haneen Ahmad, a political activist who lives in the neighborhood, said the explosion shook the area and left people sheltering in their homes in fear.

“Everyone is in panic,” Ms. Ahmad said in a phone interview. “We remained indoors till we heard the sound of the ambulances coming to the rescue.”

This is a developing story. Please check back for updates.