Afghan President Ashraf Ghani and Turkmen counterpart Gurbanguly Berdymukhamedov joined Pakistani premier Shahid Khaqan Abbasi and India's External Affairs Minister Shri M J Akbar for the ceremony at gas-rich Turkmenistan's border with Afghanistan.
Dignitaries including diplomats from the foreign missions in Turkmenistan were greeted by Turkmen national songs and traditional nomadic tents serving up food on an overcast day at the border post of Serkhetabat, once the southernmost tip of the Soviet Union.
"There were pessimistic voices, but now we are witnessing the construction of the TAPI gas pipeline," said Ghani, using the acronym for the conduit which takes its name from the four countries.
The quartet aims to complete the 1,840-kilometre (1,143-mile) pipeline and begin pumping natural gas from Turkmenistan's giant Galkynysh gas field by the beginning of 2020.
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While the pipeline will traverse war-wracked Afghanistan, raising security concerns, the bulk of the 33 billion cubic metres of gas to be pumped annually through the conduit will be purchased by South Asian rivals Pakistan and India.
And he said work on the Turkmen section of the pipeline was still ongoing.
India's commitment to the pipeline has previously been questioned over its relationship with Pakistan and easy-access to liquified natural gas markets seen as potential stumbling blocks.
But India's minister of state for external affairs M J Akbar hailed the project as "a symbol of our goals" and "a new page in cooperation" between the four countries in a speech at the ceremony today.
Turkmenistan sits on the world's fourth-largest gas reserves but lost a major buyer in Russia after Russian energy giant Gazprom wound down imports of Turkmen gas and then ceased purchases completely in 2016.
Heavy security will guard the pipeline construction through war-weary Afghanistan, AP quoted Jelani Farhad, spokesman for the Herat provincial governor's office as saying.
"It's a golden day for Afghanistan today," Farhad said. "It will help our economy and create thousands of jobs."
Security has been a leading concern leading up to the inauguration but Farhad said Afghanistan has devised a detailed security plan to protect the pipeline construction as well as the pipeline.
Taliban spokesman Zabihullah Mujahid told the AP in a telephone interview that the religious movement was ready to guarantee the pipeline's security. "We are ready to protect TAPI. It is good and important and vital for the economy of Afghanistan," he said, noting the pipeline had been under consideration during the Taliban rule that ended in 2001.
Afghan president Ghani said the next big project being readied is the laying of Afghanistan's first railway tracks, from the Turkmenistan border to Iran. He added he hoped the pipeline would signal the start of better regional cooperation.