Iranian President Mahmud Ahmadinejad and his Pakistani counterpart, Asif Ali Zardari, led a groundbreaking ceremony in the Iranian port city of Chah Bahar on March 11 to mark the start of construction on the 780-kilometer-long Pakistani segment of the pipeline.
Tehran says it has completed 900 kilometers of pipeline on its side of the border.
Iran and Pakistan hope the pipeline will be completed in time to start delivery of 21.5 million cubic meters of gas per day to Pakistan in 2014.
Last week, Zardari announced Pakistan would go ahead with the long-delayed project with Iran.
Last month, Supreme Leader Ayatollah Ali Khamenei told Zardari to ignore U.S. opposition to the project, saying Iran was the only secure source of energy that can meet Pakistan's needs.
Islamabad says the long-delayed pipeline will help alleviate the country's energy shortages.
The United States has said the project would undermine efforts to step up pressure over Tehran's nuclear program.
Washington has imposed sanctions on Iran for Tehran's failure to curb its nuclear activities, a step also taken by other Western states and the United Nations.
Rian Harris, a spokeswoman for the U.S. Embassy in Islamabad, warned that the project could result in sanctions on Islamabad.
"U.S. policy on Iran is pretty well known and has not changed and we've made it [clear] from the beginning -- since the discussions on this project have happened, we've made it clear that it is in Pakistan's interests to avoid [actions] that might be sanctionable under either UN or U.S. law," Harris told RFE/RL.
Tehran has offered to loan Pakistan one-third of the $1.5 billion the Pakistani section of the pipeline is expected to cost. Iranian contractors are to construct the pipeline in Pakistan.
The pipeline route through Balochistan Province presents significant security challenges, with separatist rebels in Pakistan's southwest fighting for autonomy and an increased share of mineral resources.
The project has faced repeated delays since talks on it began in the 1990s. The initial plan was to connect Iran's giant South Pars gas field to India via Pakistan. New Delhi withdrew from negotiations in 2009, citing costs and security issues.