The attack yesterday in the city of Qamishli took place as locals gathered at a hall to commemorate the deaths of tens of thousands of Christians by the Ottoman army starting in 1915 in what is known as the Sayfo ("Sword") massacre.
A photographer working with AFP and attending the event said he heard the blast and saw pieces of flesh lying next to damaged cars.
The security forces belonged to the Sotoro, a Christian militia based in Syria's northeast.
"Three Sotoro members were killed and five wounded," the security source said.
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One Sotoro member told AFP that the suicide bomber "detonated himself near our checkpoint after he couldn't reach his real target, Patriarch Ignatius."
Patriarch Ignatius Aphrem II is the head of the Syriac Orthodox church and was leading the commemoration.
Syria's state news agency SANA also reported three people killed in a "terrorist suicide explosion" in Qamishli but did not specify whether they were civilians or security forces.
Control of the Kurdish-majority city is split between Kurdish militia and pro-government fighters.
Syriac Christians belong to the eastern Christian tradition and pray in Aramaic. They include both Orthodox and Catholic branches, and constitute around 15 per cent of Syria's 1.2 million Christians.