Iranian President Hassan Rohani has called Pakistani Prime Minister Muhammad Nawaz Sharif to demand "serious action" to free four remaining border guards abducted by Al-Qaeda-linked militants known as Jaish Al-Adl.
Pakistan condemned the reported killing of one of the five Iranian border guards.
A statement from the Foreign Ministry said Pakistan shared "the grief and agony of the families of the martyr and the other border guards," adding that Pakistan "deeply values its brotherly relations with Iran."
It said Pakistan was giving full support to find the missing guards -- abducted in Iran's Sistan-Baluchistan Province six weeks ago -- but that investigations "have neither corroborated nor established the entry into or presence within Pakistani territory of the Iranian border guards."
Some reports said the guards are being held in Pakistan.