Pakistan has launched missile strikes into Iran, killing nine people, after Iran carried out strikes in Pakistan late on Tuesday.
Pakistan said its strikes had hit “terrorist hideouts” in Iran’s south-eastern Sistan-Baluchestan province.
Iran condemned the attack, which it said killed three women, two men and four children who were not Iranian.
The country’s foreign ministry later said it was committed to good neighbourly relations with Pakistan.
However, it called on Islamabad to prevent the establishment of “bases and armed terrorist groups” on its soil.
The reciprocal attacks come as tensions in the Middle East are high with several overlapping crises.
Israel is fighting the Palestinian group Hamas in Gaza and exchanging fire with Iran-backed Hezbollah in Lebanon.
Meanwhile, Iran-backed groups in Iraq and Syria are targeting US forces, and the US and UK have struck the Iran-backed Houthis in Yemen, who have been attacking shipping.
Thursday’s strikes by Pakistan were the first external land attack on Iran since Saddam Hussein’s forces invaded in the 1980s – launching a brutal eight-year war.
Pakistan’s foreign ministry said its strikes around the Iranian city of Saravan had come in light of “credible intelligence of impending large-scale terrorist activities” and added that it “fully respects” Iran’s “sovereignty and territorial integrity”.
In its own statement, Pakistan’s army said the “precision strikes” were conducted with drones, rockets and long-range missiles and targeted the Balochistan Liberation Army and the Balochistan Liberation Front.
Both groups are part of a decades-long struggle for greater autonomy in Balochistan, a remote region in south-western Pakistan.
Pakistan had fiercely condemned Iran’s strike on Tuesday, which struck an area of Pakistan’s Balochistan province near the Iranian border and which Islamabad said killed two children.
The country’s former foreign minister, Bilawal Bhutto Zardari, told the BBC he was surprised at the attack because Iran’s foreign minister met with Pakistan’s acting prime minister on “the day they violated the sovereignty of our country”.
He added “it would be a mistake” for a country to think Pakistan can’t respond to violations, and says it sends a “clear message that Pakistan has both the will and ability to respond”.
Iran insisted its strikes were aimed only at Jaish al-Adl, or “army of justice”, an ethnic Baloch Sunni Muslim militant group (formerly called Jundullah) that has carried out attacks inside Iran, and not Pakistan’s citizens.
Iranian state media reported on Thursday that Tehran had summoned Pakistan’s chargé d’affaires over the strikes. Pakistan had earlier recalled its ambassador and blocked the Iranian envoy from returning.
China, Turkey and the Taliban government in Afghanistan have all called for restraint and dialogue.
Earlier in the week Iran also attacked targets in Iraq and Syria. It said it had hit Islamic State and Israel’s Mossad spy agency, both of which it said had been involved in a bomb attack in the Iranian city of Kerman earlier this month which killed 84 people.
Iran and Pakistan have complicated but cordial relations. Their ministers met at Davos this week and their navies conducted joint exercises in the Strait of Hormuz and the Gulf.
The two countries have similar concerns about the lawless border area, where drug smugglers and militant Baloch groups are very active.
After both sets of air strikes, each side seemed anxious to emphasise that these did not represent attacks on a brotherly neighbour.
Tehran’s reaction to the Pakistani strike appears relatively muted and the authorities have said that the victims, who included women and children, were not Iranian nationals.
Michael Kugelman, South Asia director at the Wilson Center, said that while Pakistan’s retaliation raises the risk of escalation, “it also provides an opportunity to step back from the brink”.
“In effect, the two sides are even now. Islamabad had a strong incentive to try to restore deterrence, especially with Iran on the offensive around the wider region deploying direct strikes and proxies to hit out at threats and rivals. In effect, if Pakistan had held back, it would have faced the risk of additional strikes,” he said.
Others suggested that the government in Islamabad was under domestic pressure to respond. The country, which saw its former leader Imran Khan removed nearly two years ago, is holding an election next month.
“There was a lot of public pressure on the government to do something and so they have done this just to prove that they are not less than [Iran], this act of sabre-rattling,” said retired Lt General Asif Yaseen, a former Pakistani defence secretary.
But he said he had a “gut feeling that this will stop here for both the countries” and Pakistan could now be in a position to restart dialogue with Iran.
Some commentators have suggested Iran’s strikes on Iraq, Syria and Pakistan this week were also driven by the current turbulent dynamics in the Middle East.
Tehran has said it does not want to get involved in the wider Israel-Gaza conflict, but groups that it backs have been targeting Israel and its allies to show solidarity with the Palestinians.
However, Shashank Joshi, defence editor at The Economist, said he does not believe the strikes are an outcome of the 7 October Hamas attacks on Israel, which killed about 1,300 people and triggered Israeli retaliation against Hamas in Gaza, which officials from the Hamas-run health ministry there say has killed about 24,000 people.
“The story here is about Iran flexing its muscles, perhaps outraged by what it saw as a grievous assault on its country,” he told BBC Radio 4’s Today programme, referring to the deadly bomb attack in Kerman earlier this month, which he described as “the worst terrorist attack in Iran since the revolution of 1979”.
“Iran is wounded and is lashing out. I don’t think there’s any compelling reason to say the bombing was caused by, or is an outcome of 7 October,” he said.
He adds that this is “not the first time there have been border tensions, but it is by far and away the most serious escalation in tensions that I can remember”.
