Afghanistan, Pakistan and Turkmenistan signed a joint letter formally inviting India to join a 1,600-km natural gas pipeline project, the Asian Development Bank said on Wednesday.

The oil and gas ministers of the three countries are meeting in Manila this week to discuss logistics and funding details of the project, which is expected to take three years to build.

The ADB said the letter invited India to participate "both as an investor and as a major purchaser of the gas".

"The letter also requests the government of India to invite the Asian Development Bank to make a presentation on the project and related issues, including the approach towards mitigating risks and security concerns," it said in a statement.

The Manila-based bank, which gave a $1 million grant to finance the feasibility study of the project, said it would submit draft transportation, sale and purchase agreements to the ministers within 10 days.

The pipeline will carry up to 30 billion cubic metres of natural gas per year from the Dauletabad fields in southeastern Turkmenistan to northern India, Pakistan and Afghanistan, as well as to Arabian Sea ports for shipment to other Asian markets.

"It has significant potential to improve stability and raise living standards in South and Central Asia," the ADB said.

The next meeting of the project's steering committee -- its sixth -- is due to be held in Ashgabat, Turkmenistan at the end of May.