Tehran on Wednesday issued a warning to Islamabad after reports emerged that one of five Iranian soldiers abducted and taken across the border into Pakistan by Sunni extremists had been executed.
President Hassan Rouhani in a telephone call with Pakistan's Prime Minister Nawaz Sharif demanded "serious and swift action" by Pakistan to secure the release of the soldiers.
"We expect to hear good news in this regard," he said, while calling for "joint action by both countries against terrorists," the official IRNA news agency reported.
For his part, Sharif said the issue was of "utmost importance" to his government and that he was "prepared to boost action to free the soldiers".
Iran's Foreign Minister Mohammad Javad Zarif had earlier expressed "grave concern" about the fate of Jamshid Danayifar, who was kidnapped along with four other border guards on February 6 by rebel group Jaish-ul Adl.
"We did all we could to secure their release," Zarif told state television after a cabinet meeting.
"But it is disappointing that the Pakistani government has failed to secure its borders, and allows terrorists to operate on its soil."
Interior Minister Abdolreza Rahmani-Fazli had earlier warned -- without elaborating -- that Iran "reserves the right to utilise all its ability in its border areas."
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Jaish-ul Adl said on its website on Sunday that Danayifar had been killed, warning of further executions should Tehran refuse to "release Sunni prisoners".
The rebel group, which took up arms in 2012 to fight for what it says are the rights of Iran's minority Sunni population, is active in the restive Sistan-Baluchestan province that borders both Pakistan and Afghanistan.
In November it claimed responsibility for killing a local prosecutor, a month after its rebels killed 14 Iranian border guards in an ambush.
UN Secretary General Ban Ki-moon condemned in a Tuesday statement the reported killing as an "appalling act" and urged that the perpetrators are brought to justice.
A spokesman for the US State Department, Alan Eyre, called for the "swift release" of the abducted soldiers while expressing hope the reported execution -- that came as Iran was celebrating its Persian New Year -- was not true.
Shortly after the abduction, Iranians launched a campaign on Twitter, despite the micro-blogging service being banned in Iran.
Demanding the soldiers' release, the FreeIranianSoldiers hashtag went viral in February.
Some Iranians have used social media to hit out at the Tehran government for its inability to bring home the young soldiers, who were serving their 24-month mandatory military service.
Border guards chief Hossein Zolfaghari has admitted that there was "negligence" in the lead-up to the kidnapping, saying those responsible were suspended, with some facing prosecution.