WASHINGTON: Republicans and Democrats are pressuring Congressional negotiators to produce legislation imposing the severest penalties on Iran, targeting its energy sector and financial institutions as the United States seeks to weaken Tehran economically and derail its pursuit of nuclear weapons.

With just one week before Congress' August break, proponents of tough sanctions see this as their last, best chance for far-reaching, crippling penalties as recent high-level talks between world powers and Iran have failed to curb its uranium enrichment.

Iran insists that its program is solely for peaceful purposes.

"Now is the time to ratchet up the pressure," Rep. Ted Deutch, a Democrat, said in an interview this week.

Representative Ileana Ros-Lehtinen, the Republican chairwoman of the House Foreign Affairs Committee, and Sen. Tim Johnson, the Democratic chairman of the Senate Banking Committee, have been negotiating behind closed doors for several weeks to reach consensus on sanctions legislation that the House passed in December and the Senate approved in May.

About a half dozen Republicans and Democrats who favor fierce penalties are determined to see the toughest measure yet and have been insisting to negotiators that a watered-down bill is unacceptable.

"We owe it to the American people to exhaust every possible non-military option to prevent Iran from acquiring a nuclear weapons capability," Sen. Mark Kirk, a Republican, said in a statement.

Representative Robert Dold, a Republican, and Representative Brad Sherman, a Democrat, sent a letter this week to the negotiators calling for a final bill that includes a provision declaring Iran's energy sector "a zone of proliferation concern."

The blacklisting would bar all transactions with the state-run National Iranian Oil Company.

"This would have the effect of making virtually any transaction with — and provision of services for, the firms in Iran's energy sector sanctionable," Dold and Sherman wrote.

The lawmakers also said any legislation should include sanctions on insurance companies that knowingly provide coverage to an entity that has already been penalised.

They also are pressing for sanctions on the directors and shareholders of organisations like SWIFT, the Society for Worldwide Interbank Financial Telecommunications, unless they stop providing services to the Central Bank of Iran.