Pakistan Prime Minister Imran Khan visited Iran on Sunday following a request from the United States and Saudi Arabia for him to try to defuse rising tensions in the Gulf.
Khan's office said his visit was part of an initiative "to promote peace and security in the region", and that he would hold talks with Iran's supreme leader Ayatollah Ali Khamenei and President Hassan Rouhani.
Khan landed in Tehran and was greeted at the airport by Foreign Minister Mohammad Javad Zarif before going into a meeting with Rouhani, an AFP correspondent reported.
It is his second visit this year to Iran, which shares a border of about 1 000km with Pakistan.
Last week, a spokesperson for Pakistan's foreign office said Khan was also expected to travel to Saudi Arabia, without providing further details.
Khan said last month that both the US and Saudi Arabia had asked him to mediate with Iran to calm tensions in the Gulf.
There has been a spate of still-unexplained attacks on shipping in and around the vital seaway involving Iran and Western powers, as well as drone attacks on Saudi oil installations.
Washington accused Tehran of attacking the vessels with mines and of being behind the drone assault, something it strongly denies.
Pakistan has strong relations with Saudi Arabia, with more than 2.5 million of its nationals living and working in the kingdom.
But it also maintains good relations with Iran and represents Tehran's consular interests in the US.
Khan met both Rouhani and US President Donald Trump at the United Nations General Assembly last month, shortly after he visited Crown Prince Mohammed bin Salman in Saudi Arabia.
Iran's foreign ministry spokesperson said the day before Khan's arrival that Tehran was ready to talk with Riyadh.
"Iran has repeatedly announced that it is ready to negotiate with its neighbours including Saudi Arabia to solve any misunderstandings with our without a mediator," Abbas Mousavi was quoted as saying by the semi-official ISNA news agency.