SINGAPORE - Singapore strongly condemns the repeated brutality of militant group the Islamic State in Iraq and Syria (ISIS), the Ministry of Foreign Affairs (MFA) said in a statement on Monday.

ISIS on Sunday released a video claiming the beheading of American aid worker Peter Kassig, in the latest of a series of killings released on video by the group.

"Singapore strongly condemns the repeated brutality of the Islamic State in Iraq and Syria, most recently manifested in the murder of American aid worker Abdul-Rahman Kassig, also known as Peter Kassig," the MFA spokesman said.

"We extend our deepest condolences to the family and friends of Mr Kassig, and reiterate our call for all hostages held by ISIS to be released immediately."

"This latest incident reinforces the urgent need for the international community to work together to counter the threat posed by ISIS and other terrorist groups. We need a comprehensive strategy to contain and weaken ISIS, and tackle the roots of extremism. As a member of the multinational coalition against ISIS, Singapore will do our part to combat this terrorist scourge."

The video released on Sunday also showed the simultaneous beheadings of at least 18 men described as Syrian military personnel.