Suicide bombers attacked a pair of mosques Friday in the Yemeni capital, unleashing monstrous blasts that ripped through worshippers and killed 137 people in the deadliest assault yet targeting Shiite rebels who have taken over large parts of the rapidly fragmenting nation. At least 13 children were among the dead.

A purported affiliate of the Islamic State group claimed responsibility for the bombings, which also wounded 357 people — raising the alarming possibility the extremist group has expanded its presence to Yemen after already setting up a branch in Libya. Earlier this week, the group claimed responsibility for a bloody attack on Western tourists in Tunisia that authorities said was carried out by militants trained in Libya.

If the claim is true — and the U.S. expressed skepticism — Friday’s attacks would be the first by the Islamic State group in Yemen, adding a frightening new layer to the country’s turmoil. […]