Pakistan sent a protest letter to Iran on Saturday demanding action against Tehran-based "terrorists" involved in a recent targeted killing of its 14 security personnel, mostly from the Pakistan Navy.
Unidentified gunmen donning uniforms of paramilitary soldiers Thursday massacred 14 passengers after forcing them to disembark from buses on a highway in the restive Balochistan province.
The victims were not initially identified and it was speculated they could be Shia Muslims and ethnic workers from Punjab.
But later it emerged that they were security personnel, most of them associated with the Pakistan Navy.
According to the letter sent by the Foreign Office (FO) to Iran, at least 14 personnel belonging to the Armed Forces of Pakistan were offloaded from buses on April 18 in Oramra area of Balochistan and killed.
The Foreign Office said the "terrorists" belonging to Baloch nationalist groups were operating from a border region in Iran and has asked Tehran to take action against them.
"BRAS, which is an alliance of three Baloch terrorist organisations, has claimed responsibility for this terrorist act," it said.
"Killing of 14 innocent Pakistanis by terrorist groups based in Iran is a very serious incident that Pakistan protests strongly. Pakistan awaits Iran's response to its request for action against the groups based in Iran, whose locations have been identified by Pakistan a number of times," the letter said.
It asserted that "after the incident, the terrorists who arrived from border region returned to that area".
Baloch nationalists are active in the Balochistan province and often target the security forces and people from other provinces, especially Punjab.
The recent killing had occurred just ahead of the first-ever visit of Pakistan Prime Minister Imran Khan to Iran starting on Sunday.
Iranian foreign minister Javad Zarif condemned the attack, saying enemies of Pakistan-Iran ties were responsible for it. "Strongly condemning the recent terrorist attack in Pakistan, just as PM Imran Khan embarks on his first, historic visit to Iran.
Terrorists, extremists and their sponsors are terrified by close relations between Muslim states," he tweeted.
Balochistan, which borders Afghanistan and Iran, is Pakistan's largest and poorest province, rife with ethnic, sectarian and separatist insurgencies.
In the past, minority Shia Muslims and ethnic workers from the Punjab province have been targeted in such attacks. An ISIS suicide attack targeting Hazara Shias in the provincial capital of Quetta last week killed 21 people and injured 60 others.
Armed gunmen kidnapped about two dozen passengers from two Karachi-bound coaches from Balochistan's Mastung area in 2015, killing at least 19 of them in the mountainous area of Khad Kocha.