DUBAI - Sunni Muslim militants have killed one of the five Iranian border guards they have been holding hostage for the past six weeks, the semi-official Fars News Agency reported on Monday.It identified the victim as Jamshid Danaeefar, shown sitting against a mud wall in an unidentified location with the other four captives in a grainy photo apparently taken by their captors.Fars, quoting an "informed source", said the four other hostages were in good health, without giving further details.The guards were seized while patrolling the lawless frontier with Pakistan in early February. Jaish al-Adl (Army of Justice), an Iranian Sunni Muslim rebel group in Sistan-Baluchistan province later claimed responsibility.The group said on its website on Sunday that it had killed the border guard. Neither Tehran nor Islamabad could immediately confirm the report. Pakistani authorities said on Monday they were still investigating.Some minority Sunni Muslims in Sistan-Baluchistan, an impoverished province bordering Pakistan, have rebelled against the Shi'ite government in Tehran. The lawless province is also used by drugs and arms traffickers.