A deal has been struck on building a 1,700km (1,050m) pipeline to carry Turkmen natural gas across Afghanistan to Pakistan and India.
The Tapi project aims to feed energy-deprived South Asian markets and transit fees may benefit Afghanistan.
But details about security and funding were not addressed in the framework agreement reached by the four states.
The pipeline will have to cross Taliban-controlled regions and Pakistan's troubled border region.
Turkmenistan has previously costed the project at $3.3bn (£2.1bn, 2.5bn euros) although other estimates are as high as $10bn.
Tapi, a project which dates back to the mid-1990s, is backed by the Asian Development Bank (ADB).
The US has also encouraged the project as an alternative to a proposed Iranian pipeline to India and Pakistan.
The framework intergovernmental agreement was signed in the Turkmen capital Ashgabat by three presidents - Hamid Karzai of Afghanistan, Kurbanguly Berdymukhamedov of Turkmenistan and Asif Ali Zardari of Pakistan - and India's energy minister, Murli Deora.
"This will not be an easy project to complete - it is mandatory that we guarantee the security of the pipeline and the quality of construction work," ADP chief Haruhiko Kuroda told reporters in Ashgabat.