ISLAMABAD, Pakistan — Defying a warning from Washington, Pakistan’s prime minister promised yesterday to go ahead with a plan to import natural gas from Iran even if the United States levies additional sanctions against the Mideast country.

Prime Minister Yousuf Raza Gilani made his comments two days after the US special envoy to Pakistan, Richard C. Holbrooke, cautioned Pakistan not to “overcommit’’ itself to the deal because it could run afoul of new sanctions against Iran Congress is finalizing.

The deal has been a constant source of tension between Pakistan and the United States, with Pakistan arguing that it is vital to its ability to cope with an energy crisis and the United States stressing that it would undercut international pressure on Iran over its nuclear program.

Gilani said Pakistan would reconsider the deal if it violated UN sanctions, but the country was “not bound to follow’’ unilateral US measures. He said media reports that quoted him as saying that Pakistan would heed Holbrooke’s warning were incorrect.

The United Nations has levied four sets of sanctions against Iran for failing to suspend its uranium enrichment, a process that can produce fuel for a nuclear weapon. The latest set of UN sanctions was approved earlier this month.

The United States has also applied a number of unilateral sanctions against Iran.

Democrats and Republicans alike urged the Obama administration yesterday to endorse legislation that would significantly toughen US sanctions against Iran over its suspect nuclear program and support for extremist groups.

Senators on both sides of the aisle told two top Treasury and State Department officials that they should back a bill that targets exports of gasoline and other refined petroleum products to Iran and bans US banks from doing business with foreign banks that provide financial services to Iran’s Revolutionary Guard.

In comments to the officials testifying before the Senate Foreign Relations Committee, the lawmakers said that previous, less restrictive sanctions had failed to stop Iran from activities that could lead to nuclear weapons development. And they suggested that stability in the Middle East would be compromised unless the measures are imposed

Pakistan and Iran finalized the gas deal earlier this month. Under the contract, Iran will export 760 million cubic feet of gas per day to Pakistan through a new pipeline beginning in 2014. The construction of the pipeline is estimated to cost $7 billion.

US opposition to the gas deal has also been tempered by Washington’s reliance on Pakistani cooperation to fight Al Qaeda and Taliban militants staging attacks against NATO troops in Afghanistan.

© Copyright 2010 Globe Newspaper Company.