The international military coalition says the blast happened Wednesday but did not provide further details. While coalition deaths have risen this year, it is still rare for so many service members to die in one incident.
Two other deaths had already been reported Wednesday, making a total of six killed for the day and 34 killed so far in October.
More than 2,000 NATO service members have died since the U.S.-led invasion of Afghanistan on Oct. 7, 2001.
