Pakistan said Wednesday that Iran carried out an air strike on its territory that killed two children, after Tehran launched attacks in Iraq and Syria against what it called "anti-Iranian terrorist groups".
Pakistan denounced the strike, near the nations' shared border late on Tuesday, as "completely unacceptable", saying it was unprovoked.
Iranian news agency Mehrnews said the "missile and drone response" targeted the Jaish al-Adl group's headquarters in Pakistan, calling it "another decisive step taken by Iran in response to the aggression against the security of our country".
Formed in 2012, Jaish al-Adl is blacklisted by Iran as a terrorist group and has carried out several attacks on Iranian soil in recent years.
The strike came after Iran launched missile attacks on "spy headquarters" and "terrorist" targets in Syria, and Iraq's autonomous Kurdistan region.
The Iranian strikes add to multiple crises across the Middle East, with Israel waging a war against Hamas in Gaza and pro-Palestinian Huthi rebels in Yemen attacking commercial vessels in the Red Sea.
On Wednesday Iran's Defense minister Mohammad Reza Ashtiani said Tehran would set "no limits" to its security.
Mohammad Hosseini, deputy president for parliamentary affairs, said Pakistan had been warned "that they must prevent the entry into Iran of people who kill large numbers of people, it was natural to have the reaction of the Islamic republic".
Pakistan's official statement did not specify where the strike took place, but Pakistani media said it was near Panjgur in southwest Balochistan province, where the two nuclear-armed countries share a sparsely populated border of nearly 1,000 kilometres (620 miles).
Hours before the strike, Pakistan's caretaker Prime Minister Anwar-ul-Haq Kakar had met Iranian Foreign Minister Hossein Amir-Abdollahian on the sidelines of the World Economic Forum in Davos, Switzerland.
"This violation of Pakistan's sovereignty is completely unacceptable and can have serious consequences," Pakistan's foreign ministry said in a statement.
It said the strike caused the "death of two innocent children while injuring three girls".
Pakistan said it summoned Tehran's top diplomat in Islamabad to protest at the "unprovoked violation of its airspace".
Jaish al-Adl claimed responsibility for an attack in December on a police station in Rask that killed at least 11 Iranian police officers.
The United States has also labelled Jaish al-Adl a terrorist organisation, saying the group "primarily targets Iranian security personnel" but also government officials and civilians with assassinations, kidnappings, and suicide bombings.
Tehran and Islamabad frequently accuse each other of allowing militants to operate from the other's territory to launch attacks, but it is rare that official forces on either side engage.
"It is even more concerning that this illegal act has taken place despite the existence of several channels of communication between Pakistan and Iran," Pakistan's foreign ministry said.
"Pakistan has always said terrorism is a common threat to all countries in the region that requires coordinated action.
"Such unilateral acts are not in conformity with good neighbourly relations and can seriously undermine bilateral trust and confidence."
Michael Kugelman, director of the South Asia Institute at the Washington-based Wilson Center, warned of the seriousness of the strike.
"Iran has staged cross-border operations against Pakistan-based militants in the past, but I don't recall anything on this scale," he said on X.
"This plunges Pakistan-Iran ties -- a delicate relationship even in the best of times -- into serious crisis."
Iraq recalled its ambassador from Iran on Tuesday, condemning as a "clear act of aggression" after deadly missile strikes by its ally on its autonomous Kurdish region.
Four people were killed and six others wounded in the attack, according to the Kurdistan security council.
Iraq challenged Iran's claim that the strikes targeted Israel's intelligence services in response to recent Israeli assassinations of Iranian and pro-Iranian commanders.
It said it would lodge a complaint with the UN Security Council over the Iranian "attack on its sovereignty".
Iran defended its missile strikes in Iraq and Syria, saying they were a "targeted operation" and "just punishment" against those who breach the Islamic Republic's security.
Iran's Revolutionary Guards said they had destroyed the "Zionist regime's spy headquarters in the Kurdistan region of Iraq".
Iran has made support for the Palestinian cause a centrepiece of its foreign policy since the 1979 Islamic revolution.
It supports Palestinian militant group Hamas and hailed its deadly October 7 attacks on Israel as a "success" while denying any direct involvement.
Regional tensions have spiked since, drawing in Iran-backed armed groups in Lebanon, Iraq, Syria and Yemen.
