Tensions between nuclear-armed Islamabad, a US ally, and Washington's foe Tehran have soared after Iran struck alleged militant targets in Pakistan and Pakistan responded in kind.

The situation has also added to broader unrest in and around the Middle East, where Iran's proxies are in confrontation with Israel and the United States following the Hamas attacks on Israel on October 7.

"As you can see, Iran is not particularly well-liked in the region," Biden told reporters at the White House when asked about the clashes.

Biden said the United States was now trying to understand how the Iran-Pakistan situation would develop, adding: "Where that goes we're working on now -- I don't know where that goes."

White House National Security Council spokesman John Kirby said the United States was monitoring the situation "very, very closely" and was in touch with Pakistani officials.

"These are two well-armed nations and again we don't want to see an escalation," Kirby told journalists traveling with Biden aboard Air Force One.

Iran carried out air strikes on Tuesday, saying it had targeted a Sunni Muslim extremist group behind a series of attacks in Iranian territory. Pakistan said two children were killed.

Pakistan responded with strikes in Iran against what it said were separatist militants backing an insurgency in its restive southwestern province of Baluchistan. Tehran said nine people died and summoned Pakistan's envoy.

- 'Restraint' -

"We are concerned about escalating tensions in the region and we will continue to urge all parties to show restraint," US State Department spokesperson Matthew Miller told a briefing.

"We don't believe this should escalate in any way, shape or form. Pakistan is a major non-NATO ally of the United States, that will remain the case, but we would urge restraint in this case."

UN chief Antonio Guterres was also "deeply concerned" by the Iran-Pakistan strikes, his spokesman said.

"He urges both countries to exercise maximum restraint to avoid a further escalation of tensions," his spokesman Stephane Dujarric said.

The White House's Kirby however said Islamabad had a right to defend itself.

"They were struck first by Iran, which was obviously another reckless attack, another example of Iran's destabilizing behavior in the region," added Kirby.

Kirby said he was "not aware" that Islamabad had notified Washington before striking Iran.

He would not comment when asked if the United States would provide support for Pakistan.

Pakistan is officially a major non-NATO ally of the United States, but long had tense relations with Washington over claims that Islamabad covertly backed the Taliban in Afghanistan.

It remains a rare US partner however in a region where the Taliban have controlled Afghanistan since the chaotic US withdrawal in 2021 and where Iran holds increasing sway.

The US and Iran have been at loggerheads for decades, particularly over Tehran's nuclear program, and most recently as tensions spike over Gaza.