Pakistani President Asif Ali Zardari was due in Tehran on Saturday for talks with officials expected to include his Iranian counterpart President Mahmoud Ahmadinejad and supreme leader Ali Khamenei.
Iranian officials have given few details of the talks' agenda. Iran's ambassador to Pakistan Mashala Shokri said only that they would address "bilateral relations and regional issues."
But the visit comes less than a month after Zardari was in Tehran for a counter-terrorism summit on the sidelines of which Iran and Pakistan agreed to cooperate with Afghanistan in the fight against militants.
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Zardari's visit, on which he was to be accompanied by Interior Minister Rehman Malik, also comes after Washington announced it was deferring $800 million in military aid to Islamabad while it sought better defence cooperation.
The United States suspended the military assistance -- about one third of its $2.7 billion annual defence package -- some two months after a US operation killed Al-Qaeda leader Osama bin Laden near Pakistan's top military academy.
After the raid, the United States pledged to keep relations steady with Pakistan. But US frustration has mounted, including over Islamabad's decision to oust up to 200 US personnel who planned to train Pakistani forces.
Iranian officials have been vocal in their criticism of the prolonged US troop deployments in neighbouring Afghanistan and Iraq, both of which are now set to be drawn down.
They have also repeatedly accused the US and Pakistani intelligence services of helping Sunni Islamist insurgents active in Iran's southeastern province of Sistan-Baluchestan, which borders Pakistan's own restive Baluchistan province.