An Iranian strike on Pakistan this week that drew a rapid military riposte and raised fears of greater regional turmoil was driven by Iran's efforts to reinforce its internal security rather than its ambitions for the Middle East, according to three Iranian officials, one Iranian insider and an analyst.

Both the heavily-armed neighbours, oftentimes at odds over instability on their frontier, appear to want to try to contain the strains resulting from the highest-profile cross-border intrusions in recent years, two analysts and two of the officials said.

Iran sent shockwaves around the region on Tuesday with a missile strike against what it described as hardline Sunni Muslim militants in southwest Pakistan. Two days later, Pakistan in retaliation attacked what it said were separatist militants in Iran - the first air strike on Iranian soil since the 1980-88 Iran-Iraq war.

Tuesday's strike was one of Iran's toughest cross-border assaults on the Sunni militant Jaish al-Adl group in Pakistan, which it says has links to Islamic State. Many of Jaish's members previously belonged to a now-defunct militant group known as Jundallah that had pledged allegiance to Islamic State.

The move deepened worries about Middle East instability that have spread since the Israel-Hamas war erupted in October. Iran-allied militias from Yemen to Lebanon have launched strikes on U.S. and Israeli targets, including on Red Sea shipping, in sympathy with Gaza's Palestinians.

It also came a day after Iran launched attacks in Iraq and Syria, which it said targeted Israeli espionage and Islamic State operations, respectively.