Pakistan is pushing ahead with plans to build a gas pipeline from Iran despite strong opposition from Islamabad's strategic ally Washington, according to tender documents, Reuters reported.

State agency Inter State Gas Systems, which was set up in 1996 to oversee the development of the gas pipeline, has issued a tender inviting contractors interested in the project for pre-qualification.

"We expect that it should be about 50-60 days from the final day of submission, it will also depend on the number of applicants," a technical manager at ISGS said on condition of anonymity

"We would hope to start construction for the project by end of this year."

Another company source said the target was to see the first flow of gas from December 2016.

The project, which is estimated to cost between $1.5 to $1.8 billion, was proposed in the 1990s but has faced numerous delays. The United States has repeatedly urged Pakistan to drop the project because of its sanctions on Iran.

The document invites pre-qualification for a pipeline to transport high pressure natural gas from an off-take point near the port city of Gawadar to a gas integration point near Nawabshah.