Iran's foreign ministry on Sunday summoned the Pakistani ambassador after Tehran accused Islamabad of harbouring a militant group behind a deadly suicide attack on security forces.
"The Islamic Republic of Iran expects Pakistan's government and army to seriously confront ... the terrorist groups active on its border with Iran," the foreign ministry said in a statement.
An Iranian official at the ministry called on Pakistan to quickly take the necessary steps for the identification and arrest of the attack's perpetrators.
A Jaish al-Adl-claimed suicide bombing on Wednesday killed 27 members of Iran's Revolutionary Guards travelling on a bus in the volatile southeastern province of Sistan-Baluchestan, which straddles the border with Pakistan.
Pakistan had strongly condemned the attack and extended its deepest sympathies to the families who had lost their loved ones in the assault.
"Shocked and grieved to hear about the suicide attack on IRGC bus near Zahidan today, in which many lives were lost. Deepest sympathies with the families of those who have suffered in this heinous attack of terror. Pakistan condemns in the strongest terms all acts of terror," the Foreign Office spokesperson had said.
Iran's supreme leader Ayatollah Ali Khamenei had linked the perpetrators of the attack to "the spying agencies of some regional and trans-regional countries".
President Hassan Rouhani, vowing to avenge the attack, had said the "main root of terrorism in the region is America and Zionists, and some oil-producing countries in the region also financially support the terrorists".