The shootout between troops and suspected Islamist militants erupted on Friday evening in Sainthamaruthu in Ampara district, to the south of the town of Batticaloa, site of one of the Easter Sunday blasts at three churches and four luxury hotels.
A police spokesman said that three suspected suicide bombers were among the 15 dead after the shootout.
ISIS claimed responsibility for last Sunday’s attacks, all but one of which were in the capital, Colombo. The government has said they were carried out by nine well-educated Sri Lankans, eight of whom have been identified.
Authorities have warned there could be more attacks targeting religious centers following the bombings, which shattered the relative calm that Buddhist-majority Sri Lanka had enjoyed since a civil war against mostly-Hindu ethnic Tamil separatists ended a decade ago.
The US State Department, warning that terrorist groups were continuing to plot attacks, urged citizens to reconsider travel to Sri Lanka.