The proposed oil pipeline would run 2,615km from Iran to India through Pakistan and would carry an initial 60 million cubic metres of gas a day.
India and Pakistan said they were just weeks away from finalising terms for the cross-border pipeline on Friday, which has been hit by delays since Iran proposed it in the 1990s.
Mohammad Sadiq, Pakistan's foreign ministry spokesman, said it was an important bilateral visit, in which the transnational pipeline would be discussed, "though its technical details are being discussed separately".
The project has been dubbed the "Pipeline for Peace and Progress" because of the mutual benefits it will bring to India and Pakistan, two countries that have fought three wars since they were divided by the partition of India in 1947.
The US opposes the project because it fears it will weaken efforts to isolate Iran over its disputed nuclear programme, which Tehran insists is for power generation.
|Pakistan has said it is just weeks away from |
finalising the cross-border pipeline deal [AFP]
Sri Lanka said Iran's visit will boost the island's energy supplies and newspapers based in the capital, Colombo, welcomed Ahmadinejad's visit as a "new chapter in Lanka-Iran ties".
"Iranian president brings bounty to Sri Lanka," said the state-run Dinamina newspaper.
Mahinda Rajapakse, Sri Lanka's president, is due to greet Ahmadinejad and his 15-member delegation when they land at Bandaranaike International Airport later on Monday.
On Tuesday, he will meet leaders from all faiths in this multi-ethnic island and take part in a religious ceremony.
Iran has agreed to fund a $450m Uma Oya hydro-power project and $700m upgrade of Sri Lanka's only oil refinery.
Sri Lanka, which imports all its oil needs, of which 70 per cent is Iranian light crude, plans to triple its refinery capacity to 150,000 barrels per day.
Iran has the world's second largest reserves of gas after Russia but has been slow to develop exports partly because of US sanctions.